Some Useful Singular and Plural Forms of Medical and Anatomical Terms

Posted on 18 Oct 2012 21:08

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The plurals of medical, anatomical, and other scientific terms, having derived from Greek or Latin roots, do not follow the usual familiar rules of English.

The difference between diagnosis and diagnoses, for instance, isn't readily apparent to most people, although adding -es to the end of the word is often a way to make it plural, as we do for English words ending in z, s, x, ch, or sh. But this does not tell us what to do with scientific words like diagnosis. And there is no such word as "diagnosises." So what gives? There are some basic rules to remember to help you distinguish the singular and plural forms of medical terms. This page will run through some of these rules and list examples of each. Each rule is based on the singular form of "words ending in" a particular combination of letters.

Keep in mind, as you are using these guidelines, that they represent what is usually the rule. As with any grammar rules, they do not hold true in all instances.

words ending in is

For words ending in is, change the is to an es. Now you have your answer for diagnosis/diagnoses. Examples:

Singular Plural
Diagnosis Diagnoses
Neurosis Neuroses
Prosthesis Prostheses

An exception has been noted in the word epidydimis, which becomes epidydimides when plural.

words ending in us

This rule does not work in all instances but it works for many words ending in us: change the us to an i. Examples:

Singular Plural
Anulus Anuli
Bacillus Bacilli
Bronchus Bronchi
Calculus Calculii
Embolus Emboli
Fundus Fundi


Certain words that end in us are pluralized by changing the us to era. For example, viscus becomes viscera. Other words have the us change to ora, such as corpus to corpora. However, we hardly ever use the singular form of viscera and corpus is not really used at all except by authors who rely too much on a thesaurus, or very religious ones.

There are some other notable exceptions. The plural of virus is viruses and the plural of sinus is sinuses.

The suffix -us is an ending that is used for singular nouns.

words ending in a

Words ending in a are made plural by adding an e to the end, but keeping the a.

Singular Plural
Scapula Scapulae
Papilla Papillae
Sclera Sclerae
Larva Larvae
Petechia Petechiae

If there is a Latin modifier with the a word, it must agree as well. So vena cava becomes venae cavae.

The suffix -a is a common noun ending.

words ending in um

Words ending in um are pluralized by dropping the um and adding an a. Examples:

Singular Plural
[* Acetabulum] Acetabula
Atrium Atria
Bacterium Bacteria
Capitulum Capituli
Septum Septi
Diverticulum Diverticuli

The suffix -um is a singular noun ending.

words ending in ex, ix. or yx

To make words ending in ex, ix, or yx plural, replace the endings with ices. Examples:

Singular Plural
Appendix Appendices
Apex Apices
Calix Calices
Cervix Cervices
Index Indices
Varix Varices

words ending in oma

For words ending in oma, keep the oma and add a ta or an s. Both forms are commonly used, but for most words, one form is more common than the other and the s form is probably the most commonly used, in general. Examples:

Singular Plural
Adenoma Adenomata, Adenomas
Carcinoma Carcinomata, Carcinomas
Condyloma Condylomata, Condylomas
Fibroma Fibromata, Fibromas

words ending in nx

For words ending in nx, drop the x and add ges. Examples:

Singular Plural
Larynx Larynges
Phalanx Phalanges

words ending in on

To pluralize words ending in on, drop the on and add an a. Sometimes, as well, an s is simply added to the end of the on, but do not use this indiscriminately. For instance, the familiar word phenomena is also a scientific one, and you should never say phenonenoms, as this just sounds wrong. Phenomena is correct. In other cases, the s ending has begun to be used but it is best to stick to the a ending when in doubt. Examples:

Singular Plural
Ganglion Ganglia, Ganglions
Criterion Criteria, Criterions (rare)

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