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Plurals of English nouns taken from Latin or Greek

Words with two plurals

English has borrowed many common words from Latin and Greek, such as amoeba, appendix, aquarium, criterion, referendum, syllabus, and virus. In their original language the plurals of such words followed that language’s rules: the Latin plural of aquarium is aquaria, the Greek plural of criterion is criteria, and so forth. However, most of these words also form regular plurals in English by adding an -s, e.g. aquariums, referendums, syllabuses.

Which form to use?

As a rule of thumb, the Latin-style plural is appropriate to formal, scientific, or technical writing, while the English plural is better suited to everyday language. Rock guitarists use plectrums, and might be ridiculed if they called them plectra.

Choosing to use the Latin plural form when an English one is also available can smack of pretentiousness or pomposity, as when talking of online fora rather than forums.

Common words with both forms

When deciding which to choose, it is important to bear in mind the context in which it will appear, as outlined above.

SINGULAR	PLURAL
addendum	addendums or addenda
aquarium	aquariums or aquaria
gymnasium	gymnasiums or gymnasia
maximum	maximums or maxima
memorandum	memorandums or memoranda
minimum	minimums or minima
moratorium	moratoriums or moratoria
referendum	referendums or referenda

Though both curricula and curriculums are used, the first is found rather more frequently.

Common words with plural in -uses

SINGULAR	PLURAL
caucus	caucuses
f(o)etus	f(o)etuses
hiatus	hiatuses
phallus	phalluses
prospectus	prospectuses
sinus	sinuses
virus	viruses

The word ignoramus is not a Latin noun, so its plural can only ever be ignoramuses. Both syllabuses and syllabi are used, but the first is used more often in World English, while the second is often used in American English.

The plural of octopus is another special case.

Latin plurals only

The following words always take the Latin plural:

• alga => algae
• desideratum => desiderata
• larva => larvae
• nucleus => nuclei

 Special cases

For a very small group of words, the choice of plural depends on the subject field: e.g. appendixes in surgery and zoology but appendices in books. In scientific work foci, formulae, indices, and vortices are regularly used, but in general writing the ordinary plural forms in -s and -es are more usual.

Be careful with...

Three words have plurals which are regularly used as singulars in a way that many people will consider a mistake:

SINGULAR	PLURAL
criterion	criteria
stratum	strata
phenomenon	phenomena

You should therefore avoid phrases such as these:

X a strata of society

X a strange phenomena

People also disagree about whether data is singular or plural, as explained in the note here.