Introduction to Medical Terminology (The Ultimate Guide)

A Beginners Guide to your Medical Terms Journey

Medical terminology is the language of medicine, it is a way of describing the human body and its components, processes, conditions, and procedures.  It is based on the terminology of Greek and Latin, it is uniform throughout the world.  Medical terminology is vast and mastering it can be like learning an entire foreign language. 

Learning Medical Terminology can be a daunting task, but there are ways to remember these medical words and can help in making informed guesses.  Medical terms can be divided into root words, prefixes and suffixes. Each retains its meaning when appearing as part of a medical term. By knowing the component parts of medical terms, you can remember many medical words.

What is the purpose of Medical Terminology?

A medical terminology system is a way to create a common language for medical professionals. This language helps medical staff communicate more effectively and it makes documentation easier.

Medical terminology allows medical professionals to communicate more easily with each other, and with other medical professionals allowing them to be more effective in their practice.

When is Medical Terminology used?

Medical terminology is used across the healthcare industry. Medical care is documented using it to ensure that every medical decision and procedures is documented for physicians, nurses, and other medical professionals to collaborate effectively about medical issues.

Medications uses medical terms as well with other variety of medical settings, such as in patient care, lab tests, and more.

Who needs to learn Medical Terminology?

Medical terminology is essential for anyone who works in an environment where a basic understanding of medical terms is required.

Along with some popular healthcare professionals, such as doctors, nurses, radiologists, and pharmacists. The knowledge of medical terminologies is also required for health care assistants, medical transcriptionists, medical records clerks, medical coders, and billers.

Why is it important for health professionals to use Medical Terminology?

Doctors and other health professionals are expected to communicate frequently, so they need to become familiar with medical terms. Medications, procedures, treatments, and medical conditions are often described using medical terms.

Healthcare professionals need to understand medical terms so that they can convey accurate information to patients as well.

Without medical terminology, doctors, nurses, and other healthcare providers would be at risk of misdiagnosing or prescribing the wrong medication. Also, they would make mistakes during medical procedures and health care related procedures.

Medical terminology is the language of medicine. It is the foundation of any successful healthcare profession.

Basic Structure of Medical Terminology

Medical terms are made up of word parts. Prefix, word root, suffix, and combining form vowel are these word parts.

Word Parts

  1. The prefix is a word part added to the beginning of a medical term that alters the meaning of the root. The prefix usually indicates direction, type, location, quality, or quantity. ​​
  2. The word root (also known as the root) is the fundamental unit of every medical term. It identifies a word’s basic meaning and is the part to which modifying prefixes and suffixes are added.
  3. The suffix is a word part that is added to the end of a word root that changes it’s meaning.
  4. Combining forms vowels are used to connect word parts and to make pronunciation easier. In most cases, combining form vowels are an “o,” but sometimes they are “i” or “e.”

You can identify the meaning of medical terms or words by breaking them down into word parts. Here is an example of the medical word broken down into its parts.

This image shows a sample of a word breakdown of a medical word. It shows a medical word that is composed of a prefix, root and suffix.

Word Root

The Word Root is the foundational meaning of a medical term. Roots contain the essence of the word, and they are the part to which prefixes and suffixes are added for modifying meanings. Word roots for medical terms often describe an organ, tissue, or condition. Almost every medical term has one or more word roots.

Hypotension, for example, has the prefix “hypo-” meaning “low” or “below”, and the root word “tension” means “pressure”, so the word “hypotension” denotes abnormally low blood pressure.

Combining Form

In order to facilitate pronunciation, a vowel (mostly a letter o) is usually inserted between the root and the suffix when a suffix begins with a consonant.

Illustration of a combining form.

The word part resulting from the combination of a root and a combining form vowel is an example of a combining form. In textbooks, you will notice that combining forms show a slash between the root and the vowel, as in cardi/o.

Basically, combining forms are the combination of word roots + combining form vowels. Simply put, combining form is also a root word that has a combining form vowel on it.

*Most textbooks automatically call Combining Form a Word Root as well. 

Compound Words

A compound word is a medical term with multiple roots. This is often the case when referring to more than one body part or system. 

Illustration of a compound word.
Both osteo and arthr are Word Root in one single medical term.

Common Word Roots

Below are lists of common word roots categorized by each system.


Common Word Roots for Cells & Tissues

Word Root Combining Form Body Part or Condition
aden aden/o gland
cyt cyt/o cell
fibr fibr/o fiber
hist, histi hist/o, histi/o tissue
morph morph/o form
myx, muc myx/o, muc/o mucus
nucle, kary nucle/o, kary/o nucleus
papill papill/o nipple
reticul reticul/o network
somat somat/o body

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Common Word Roots for Disease

Word Root Combining Form Body Part or Condition
alg, algi, algesi alg/o, algi/o, algesi/o pain
carcin carcin/o cancer
lith lith/o stone
onc onc/o tumor
path path/o disease
py py/o pus
pyr, pyret pyr/o, pyret/o fever
therm therm/o heat
tox, toxic tox/o, toxic/o poison or toxin

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Common Word Roots for Cardiovascular System

Word Root Combining Form Body Part or Condition
angi, vas, vascul angi/o, vas/o, vascul/o vessel (usually blood vessel)
aort aort/o aorta
arter, arteri arter/o, arteri/o artery
arteriol arteriol/o arteriole
ather ather/o fatty plaque
atri atri/o atrium
cardi cardi/o heart
hemat, hem hemat/o, hem/o blood
isch isch/o blockage
myel myel/o bone marrow (also refers to spinal cord)
plasm plasm/o plasma
thromb thromb/o clot
valv, valvul valv/o, valvul/o valve
ven, ven, phleb ven/o, ven/i, phleb/o vein
ventricul ventricul/o ventricle

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Common Word Roots for Lymphatic System

Word Root Combining Form Body Part or Condition
adenoid adenoid/o adenoids
immun immun/o immune or immunity
lymph lymph/o lymph or lymphatic system
lymphaden lymphaden/o lymph gland or lymph node
lymphangi lymphangi/o lymphatic vessel
myel myel/o bone marrow (also refers to spinal cord)
splen splen/o spleen
thym thym/o thymus gland
tonsill tonsill/o tonsil

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Common Word Roots for Respiratory System

Word Root Combining Form Body Part or Condition
alveol alveol/o alveolus
bronch, bronch, bronchi bronch/o, bronch/i, bronchi/o bronchus, bronchial tube
bronchiol bronchiol/o bronchiole
capn capn/o carbon dioxide
epiglott epiglott/o epiglottis
laryng laryng/o larynx
lob lob/o lobe
muc muc/o mucus
nas, rhin nas/o, rhin/o nose
ox ox/i oxygen
pharyng pharyng/o pharynx
phon, son phon/o, son/o sound, voice
phren, diaphragmat phren/o, diaphragmat/o diaphragm
phrenic phrenic/o phrenic nerve
pleur pleur/o pleura
pulm, pulmon, pneumon, pneum, pneumat pulm/o, pulmon/o, pneumon/o, pneum/o, pneumat/o lungs or respiration
py py/o pus
radi radi/o x-rays or ionizing radiation
sept sept/o septum
sinus sinus/o sinus
spir, respir spir/o, respir/o breathing
thorac thorac/o thorax or chest cavity
trache trache/o trachea

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Common Word Roots for Digestive System

Word Root Combining Form Body Part or Condition
abdomin , celi, lapar abdomin/o , celi/o, lapar/o abdomen or abdominal
an an/o anus
antr antr/o antrum
append, appendic append/o, appendic/o appendix
bil, chol bil/i, chol/e, chol/o bile, gall
bucc bucc/o cheek
cec cec/o cecum
cholangi/o cholangi/o bile duct
cholecyst cholecyst/o gallbladder
choledoch choledoch/o common bile duct
col, colon col/o, colon/o colon
dent, dent, odont dent/o, dent/i, odont/o tooth or teeth
diverticul diverticul/o diverticulum
duoden duoden/o duodenum
enter enter/o intestine
esophag esophag/o esophagus
gastr gastr/o stomach
gingiv gingiv/o gum
gnath gnath/o jaw
hepat hepat/o liver
herni herni/o hernia
ile ile/o ileum
jejun jejun/o jejunum
labi, cheil labi/o, cheil/o lip
lingu, gloss lingu/o, gloss/o tongue
or, stoma, stomat or/o, stoma, stomat/o mouth
palat palat/o palate
pancreat pancreat/o pancreas
peritone peritone/o peritoneum
pylor pylor/o pylorus or pyloric sphincter
rect, proct rect/o, proct/o rectum
sial sial/o saliva, salivary gland or salivary duct
sigmoid sigmoid/o sigmoid colon
steat steat/o fat
uvul uvul/o uvula

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Common Word Roots for Urinary System

Word Root Combining Form Body Part or Condition
albumin albumin/o albumin
azot azot/o urea; nitrogen
calic, cali calic/o, cali/o calyx
cyst, vesic cyst/o, vesic/o urinary bladder
glomerul glomerul/o glomerulus
hydr hydr/o water
lith lith/o stone or calculus
meat meat/o meatus
pyel pyel/o renal pelvis
ren, nephr ren/o, nephr/o kidney
ur, urin ur/o, urin/o urine or urinary tract
ureter ureter/o ureter
urethr urethr/o urethra

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Common Word Roots for Male Reproductive System

Word Root Combining Form Body Part or Condition
andr andr/o male
balan balan/o glans penis
epididym epididym/o epididymis
gonad gonad/o gonad
osche, scrot osche/o, scrot/o scrotum
pen, phall pen/o, pen/i, phall/o penis
preputi, posth preputi/o, posth/o prepuce (foreskin)
prostat prostat/o prostate gland
semin semin/i semen
sperm, spermat sperm/o, spermat/o spermatozoon (sperm)
test, testicular, orch, orchi, orchid test/o, testicular/o, orch/o, orchi/o, orchid/o testis (testicle)
urethr urethr/o urethra
vas vas/o vas deferens (also refers to vessel)
vesicul vesicul/o seminal vesicle

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Common Word Roots for Female Reproductive System

Word Root Combining Form Body Part or Condition
amni amni/o amnion (amniotic sac)
cervic, trachel cervic /o, trachel/o cervix (also refers to neck)
clitor, clitorid clitor/o, clitorid/o clitoris
embry embry/o embryo
endometri endometri/o endometrium
fet fet/o fetus
gravid gravid/o pregnant woman
gyn, gynec gyn/o, gynec/o woman
hymen hymen/o hymen
lact, galact lact/o, galact/o milk
mamm, mast mamm/o, mast/o breast (mammary gland)
men men/o menstruation
nat nat/i birth
oo, ov oo, ov/o, ov/i ovum, egg cell
ovari, oophor ovari/o, oophor/o ovary
para para has given birth
pelv pelv/i pelvis, pelvic bones or pelvic cavity
perine perine/o perineum
salping salping/o uterine tube or fallopian tube
toc toc/o labor
uter, metr, hyster uter/o, metr/o, metr/i, hyster/o uterus
vagin, colp vagin/o, colp/o vagina
vulv, episi vulv/o, episi/o vulva

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Common Word Roots for Endocrine System

Word Root Combining Form Body Part or Condition
aden aden/o gland
adren, adrenal adren/o, adrenal/o adrenal gland or epinephrine
adrenocortic adrenocortic/o adrenal cortex
calc calc/i calcium
endocrin endocrin/o endocrine glands or system
insul insul/o pancreatic islets
kal kal/i potassium
natr natr/o sodium
parathyr, parathyroid parathyr/o, parathyroid/o parathyroid gland
pituitar, hypophys pituitar/o, hypophys/o pituitary gland (hypophysis)
thyr, thyroid thyr/o, thyroid/o thyroid gland

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Common Word Roots for Nervous System

Word Root Combining Form Body Part or Condition
cephal cephal/o head
cerebell cerebell/o cerebellum
cerebr cerebr/o cerebrum
cortic cortic /o cerebral cortex
dur dur/o dura mater
encephal encephal/o brain
esthesi esthesi/o sensation
gangli, ganglion gangli/o, ganglion/o ganglion
gli gli/o neuroglia
medull medull/o medulla oblongata
mening, meningi mening/o, meningi/o meninges
myel myel/o spinal cord
narc narc /o stupor; numbness
neur, neur neur/o, neur/i nervous system or nerve
phas phas/o speech
poli poli/o gray matter
psych, ment psych/o, ment/o mind
radic, radicul, rhiz radic/o, radicul/o, rhiz/o nerve root
somn, somn somn/o, somn/i sleep
thalam thalam/o thalamus

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Common Word Roots for Sensory System

Word Root Combining Form Body Part or Condition
acoust, acous acoust/o, acous/o sound or hearing
audi audi/o hearing
chori, choroid chori/o, choroid/o choroid
cochle cochle/o cochlea
conjunctiv conjunctiv/o conjunctiva
corne, kerat corne/o, kerat/o cornea or horn
cycl cycl/o ciliary body or ciliary muscle
dacryocyst dacryocyst/o lacrimal sac
ir, irit, irid ir/o, irit/o, irid/o iris
labyrinth labyrinth/o labyrinth (inner ear)
lacrim, dacry lacrim/o, dacry/o tear or tear duct
lent, phak, phac lent/i, phak/o, phac/o lens
mastoid mastoid/o mastoid bone
myring, tympan myring/o, tympan/o tympanic membrane (eardrum)
opt, ocul, ophthalm opt/o, ocul/o, ophthalm/o eye
ot, aur, aur ot/o, aur/o, aur/i ear
palpebr, blephar palpebr/o, blephar/o eyelid
phot phot/o light
pupill, core, cor pupill/o, core/o, cor/o pupil
retin retin/o retina
salping salping/o auditory eustachian tube (also refers to the fallopian tube of the uterus)
scler scler/o sclera
staped, stapedi staped/o, stapedi/o stapes (middle ear bone)
uve uve/o uvea
vestibul vestibul/o vestibule of the inner ear

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Common Word Roots for Skeletal System

Word Root Combining Form Body Part or Condition
ankyl ankyl/o stiff, bent
aponeur aponeur/o aponeurosis
arthr arthr/o joint
burs burs/o bursa
carp carp/o carpals, wrist
chondr chondr/o cartilage
clavic, clavicul clavic/o, clavicul/o clavicle (collarbone)
coccy, coccyg coccy, coccyg/o coccyx
cost cost/o rib
crani crani/o skull or cranium
disk disk/o intervertebral disk
femor femor/o femur (upper leg bone)
fibul fibul/o fibula (outer lower leg bone)
humer humer/o humerus (upper arm bone)
ili ili/o ilium
ischi ischi/o ischium
kinesi kinesi/o movement (motion)
kyph kyph/o humpback
lord lord/o curve; swayback
lumb lumb/o loin (lumbar region of the spine)
mandibul mandibul/o mandible (lower jawbone)
maxill maxill/o maxilla (upper jawbone)
menisc menisc/o meniscus
myel myel/o bone marrow (also refers to the spinal cord)
myel myel/o spinal cord (also refers to the bone marrow)
oste oste/o bone
patell patell/o patella (kneecap)
pelv, pelv pelv/i, pelv/o pelvis (pelvic bone)
phalang phalang/o phalanges (finger and toes bones)
pub pub/o pubis
rachi rachi/o vertebrae (spinal column)
radi radi/o radius (lateral forearm bone)
sacr sacr/o sacrum
scapul scapul/o scapula (shoulder blade)
scoli scoli/o curved
spondyl, vertebr spondyl/o, vertebr/o vertebra
stern stern/o sternum (breastbone)
synov synov/i synovial fluid, joint, or membrane
tars tars/o tarsals (ankle bones)
ten, tendin, tend ten/o, tendin/o, tend/o tendon
tibi tibi/o tibia (inner lower leg bone)
uln uln/o ulna (medial forearm bone)

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Common Word Roots for Muscular System

Word Root Combining Form Body Part or Condition
fasci fasci/o fascia
flex flex/o bend
in in/o, fibr/o fiber
kin, kinesi, kinet kin/e, kinesi/o, kinet/o movement
leiomy leiomy/o smooth visceral muscle
my, myos, muscul my/o, myos/o, muscul/o muscle
myocardi myocardi/o cardiac muscle
rhabdomy rhabdomy/o skeletal or striated muscle
sarc sarc/o connective tissue; soft or flesh
ten, tendin ten/o, tendin/o, tend/o tendon
tens tens/o stretch; strain
ton ton/o tone

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Common Word Roots for Integumentary System

Word Root Combining Form Body Part or Condition
derm, dermat, cutane derm/o, dermat/o, cutane/o skin
hidr, idr hidr/o, idr/o sweat
hirsut hirsut/o hairy, rough
melan melan/o melanin or dark
myc myc/o fungus
necr necr/o death
onych, ungu onych/o, ungu/o nail
pachy pachy/o thick
rhytid rhytid/o wrinkles
seb seb/o sebum or sebaceous gland
staphyl staphyl/o grapelike clusters (also refers to uvula)
strept strept/o twisted chains or strips
trich trich/o hair
xer xer/o dry

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Prefixes

The prefix is a word part added to the beginning of a medical term that alters the meaning of the word root.  A prefix can describe the quantity, quality, location, color, direction, time, degree, or status of a word root. Medical terms do not all have prefixes.

*When written alone or detached from other medical word parts, the prefix is sometimes followed by a hyphen (-). For example, in the medical term hypertension, (hyper-) is written with a hyphen to distinguish it as a prefix

Common Prefixes

A list of commonly used prefixes is provided below, categorized accordingly.


Prefixes that Describe the Quantity or Number

Prefix Number
nulli- none
semi-, hemi- half or partial
poly-, multi- many
prim(i)- first
mon(o)-, uni-, hapl(o)- one, single
di-, dipl(o)-, bi- two, double
tri- three
quadr(i)-, tetr(a)- four
pent(a)- five
hex(a)- six
hept(a)- seven
oct(o)-, oct(a)- eight
nona-,noni- nine
deci-, deca- ten
cent(i)- hundred
milli- thousand

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Prefixes that Describe the Color

Prefix Color
chlor(o)- green
cirrh(o)-, jaund(o)-, xanth(o)- yellow
cyan(o)- blue
erythr(o)-, rubr(o)-, rhod(o)-, eosin(o)- red
glauc(o)-, poli(o)- grey or silver
leuk(o)-, alb-, albin(o)- white
melan(o)- black
purpur-, porphyr- purple

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Prefixes that Describe the Size

Prefix Size
micr(o)- small
macr(o)-, mega(lo)- large
iso-, equi- equal

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Prefixes that Describe the Degree

Prefix Degree
hyp(o)-, sub- low, below normal
hyper-, super-, ultra- high, above normal
norm(o)- normal

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Prefixes that Describe the Position or Direction

Prefix Position or Direction
ab- away
ad- toward
ambi- both
ante-, anter(o)-, ventr(i)-, ventr(o)- anterior (front)
dextr(o)- right
ect(o)-, ex(o)-, extra-, extro- out, outside
end(o)-, intra-, intro- inside
ep(i)-, supra-, super- above
inter- between
later(o)- lateral or side
mes(o)-, medi(o)-,medi(a)- middle
juxta- near
para- near, apart from, abnormal
peri- , circum- around
poster(o)-, dors(i)-, dors(o)- posterior (back)
sinistr(o)-, levo- left
sub-, infra- below
tele-, telo- distant
trans-, dia-, per- through

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Prefixes that Describe the Time or Speed

Prefix Time or Speed
ante-, pre-, pro- before
brady- slow
chron(o)- time
neo- new, recent
post- after
re- again
retr(o)- back, backward
tachy- fast

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Prefixes that Describe the Status

Prefix Status
a-, an-, non-,un- not, without
anti-, contra- against
de- away from, without
dis- apart, separate
eu- normal
heter(o)- other, different
hom(o)- same, similar
mal- bad
dys- bad, difficult, painful, abnormal
orth(o)- straight; correct
poikilo- irregular, varied
pseud(o)- false

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Suffix

The use of suffixes changes the meaning of medical terminology. It is added to the end of a medical word to typically describe procedures, conditions, functions or just to make a medical word a noun or an adjective.

 For example in the word arthritis which means inflammation of the joints, the word root “arthr” means joints while the suffix “-itis” means inflammation. Like the prefix, the suffix when detached in a medical term is written with a hyphen (-) before the word part as demonstrated for the suffix “-itis“.

In other cases, the suffixes have no real meaning to them and their sole purpose is to make or form the medical term into a noun or an adjective. A good example is the suffix -um of the word endocardium. The prefix endo- means inside, the root cardi means heart, and the suffix -um makes this medical term turn into a noun.

Common Suffixes

The following are examples of suffixes that are categorized accordingly.


Suffixes that Describe the State or Form

Suffixes that Forms Noun Meaning
-a, -e, -um, -is noun ending with no specific meaning
-gen, genic causing
-ia, -ism, iasis, -sis, -y condition
-ian,- iatrics, -iatry, -ics,- ist, -logy specialty
Suffixes that Forms Adjective Meaning
-ac,-al, -ar(y), -form, -ic(al), -ile, -ior, -ory,-ous, -tic pertaining to
-form, -oid resembling

Suffixes that Describe Surgery, Diagnosis & Procedure

Suffix Meaning
-(a)pheresis removal
-centesis surgical puncture
-desis fusion of two parts into one
-ectomy surgical removal
-gram the record
-graph instrument used to record
-graphy process of recording
-metry, -meter measurement
-opsy looking at
-ostomy, -stomy surgically creating a hole
-otomy, -tomy surgical incision
-tome surgical incision instrument or section; segment
-pexy fix or secure
-plasty surgical repair or reconstruction
-rrhaphy surgical suturing
-scope instrument used for viewing or examination
-scopic pertaining to visual examination
-scopy process of viewing with a scope
-tripsy crushing or breaking up

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Suffixes that Describe Disease or Function

Suffix Disease or Function
-algia, -algesia, -dynia pain
-asthenia weakness
-carcinoma cancerous tumor
-cardia heart condition
-cele hernia, swelling
-clasis, -clasia breaking
-cyesis pregnancy
-drome running
-ectasia, -ectasis, -dilation, -dilatation dilation
-edema swelling
-emesis vomiting
-emia blood or blood condition
-itis inflammation
-lysis separating, dissolution
-malacia softening
-megaly enlargement
-necrosis death of tissue
-oma tumor
-opia vision
-osis abnormal condition
-paresis slight paralysis
-pathy disease
-penia abnormal reduction in number
-pepsia digestion
-phagia swallowing, eating
-phasia speech
-pheresis removal
-phil(ia) affinity for, attraction
-phobia irrational fear
-phoresis transmit
-physis growth
-plasia formation, growth
-plasm living substance, tissue
-plegia paralysis
-pnea breathing
-poiesis formation
-ptosis prolapse, drooping
-rrhage, -rrhagia excessive bleeding
-rrhea flow, discharge
-rrhexis rupture
-salpinx fallopian tube, uterine tube
-sarcoma malignant tumor
-schisis fissure, splitting
-sclerosis hardening
-spasm sudden, involuntary contraction of muscle
-stalsis contraction
-stasis control, stop
-stenosis tightening
-thorax chest, chest cavity
-tocia labor, birth
-trophy nourishment; development
-uria urine, urination

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Plural Forms

Medical terms can have different rules regarding forming plural terms than the standard English terms, where we use es and s in the ending of the word. Following are the rules that are used in transforming singular medical terms to plurals:

Rule 1: For singular ending a, just add e to make it plural.

Singular Form Plural Form
areola areolae
macula maculae
retina retinae
trachea tracheae
vertebra vertebrae

Rule 2: For singular ending with ax , change the ax to aces.

Singular Form Plural Form
anthrax anthraces
pneumothorax pneumothoraces
thorax thoraces

Rule 3: For singular ending with ex or ix, change the ex or ix to ices.

Singular Form Plural Form
cortex cortices
subcortex subcortices
appendix appendices
cervix cervices
helix helices

Rule 4: For singular ending with is, change the is to es.

Singular Form Plural Form
cholelithiasis cholelithiases
diabetis diabetes
diagnosis diagnoses
emesis emeses
osteoporosis osteoporoses

Rule 5: For singular ending ma, just add ta to make it plural.

Singular Form Plural Form
blastoma blastomata
edema edemata
fibroma fibromata
sarcoma sarcomata
stigma stigmata
stoma stomata

Rule 6: For singular ending nx (anx, inx, ynx), change nx to nges.

Singular Form Plural Form
larynx larynges
meninx meninges
phalanx phalanges
pharynx pharynges
salpinx salpinges

Rule 7: For singular ending on, change on to a.

Singular Form Plural Form
ganglion ganglia
myelencephalon myelencephala
spermatozoon spermatozoa

Rule 8: For singular ending um, change um to a.

Singular Form Plural Form
atrium atria
bacterium bacteria
clostridium clostridia
diverticulum diverticula
duodenum duodena
endocardium endocardia
endometrium endometria
ostium ostia
ovum ova
reticulum reticula

Rule 9: For singular ending us, change us to i.

Singular Form Plural Form
bacillus bacilli
calculus calculi
canthus canthi
embolus emboli
malleus mallei
meniscus menisci
nucleus nuclei
phallus phalli
pylorus pylori
radius radii
thrombus thrombi

Rule 10: For singular ending y, change y to ies.

Singular Form Plural Form
arthroscopy arthroscopies
biosy biopsies
cavity cavities
epilepsy epilepsies
laryngoscopy laryngoscopies
microscopy microscopies
narcolepsy narcolepsies
retinoscopy retinoscopies
therapy therapies

Exemptions

Even though there are these rules for medical terms, you will notice in dictionaries that the standard English rules like using -s and -es are also being used as acceptable plural forms in several medical terms. For example, the word retina is pluralized as retinae when following the rules above, but the word retinas is also acceptable. 

A second example is the word virus, where “viruses” is the correct plural form instead of “viri” not adhering to the rules above which states that singular form ending with the letter -us should be changed to -i