Some words borrowed from other languages, in particular Latin, Greek and French, have irregular plurals in English. Although many now have the usual English ‑s plural, some still retain a Latin, Greek or French plural ending, while others have both. For example, the plural of stimulus is stimuli, the plural of focus is focuses or foci, and the plural of virus is viruses.
Your computer spelling checker will alert you to incorrect plural forms, though in some cases preference for the original or the English ‑s plural differs from subject to subject and between formal and informal language.
Examples of the most common types of irregular plurals in university and research writing are given below.
-a to -ae
| Singular | Plural |
| antenna | antennae or antennas |
| formula | formulas or formulae |
| larva | larvae |
| nebula | nebulae or nebulas |
| vertebra | vertebrae or vertebras |
-eau to -eaux
| Singular | Plural |
| bureau | bureaus or bureaux |
| plateau | plateaus or plateaux |
| tableau | tableaux |
-ies and -ies
| Singular | Plural |
| series | series |
| species | species |
-is to -es
| Singular | Plural |
| analysis | analyses |
| axis | axes |
| basis | bases |
| crisis | crises |
| diagnosis | diagnoses |
| hypothesis | hypotheses |
| metamorphosis | metamorphoses |
| nemesis | nemeses |
| neurosis | neuroses |
| oasis | oases |
| parenthesis | parentheses |
| synopsis | synopses |
| thesis | theses |
-ma to -mata
| Singular | Plural |
| angioma | angiomas or angiomata |
| carcinoma | carcinomas or carcinomata |
| h(a)ematoma | h(a)ematomas or h(a)ematomata |
| lymphoma | lymphomas or lymphomata |
| melanoma | melanomas also melanomata |
| sarcoma | sarcomas or sarcomata |
| schema | schemata or schemas |
| stigma | stigmas and stigmata |
| stoma | stomata or stomas |
-o to -oes
Not all words ending in -o add -e- in the plural, for example embryos, neutrinos, placebos. However, a number of those that do are listed below:
| Singular | Plural |
| archipelago | archipelagos or archipelagoes |
| cargo | cargos or cargoes |
| echo | echoes |
| embargo | embargoes |
| fresco | frescos or frescoes |
| ghetto | ghettos or ghettoes |
| halo | haloes |
| hero | heroes |
| mango | mangos or mangoes |
| manifesto | manifestos or manifestoes |
| mosquito | mosquitoes or mosquitos |
| potato | potatoes |
| tomato | tomatoes |
| tornado | tornadoes |
| veto | vetoes |
| volcano | volcanoes |
-on to -a
| Singular | Plural |
| criterion | criteria |
| ganglion | ganglia or ganglions |
| mitochondrion | mitochondria |
| phenomenon | phenomena |
| spermatozoon | spermatozoa |
| taxon | taxa |
-um to -a
| Singular | Plural |
| addendum | addenda |
| aquarium | aquariums or aquaria |
| auditorium | auditoriums or auditoria |
| bacterium | bacteria |
| consortium | consortia also consortiums |
| continuum | continua or continuums |
| curriculum | curricula or curriculums |
| datum | data |
| dictum | dicta or dictums |
| equilibrium | equilibriums or equilibria |
| erratum | errata |
| forum | forums or fora |
| maximum | maximums or maxima |
| medium | media |
| memorandum | memorandums or memoranda |
| millennium | millennia or millenniums |
| minimum | minimums or minima |
| moratorium | moratoria or moratoriums |
| ovum | ova |
| phylum | phyla |
| referendum | referenda or referendums |
| septum | septa |
| serum | serums or sera |
| spectrum | spectra or spectrums |
| stratum | strata or stratums |
| symposium | symposia or symposiums |
| ultimatum | ultimatums or ultimata |
-us to -i
| Singular | Plural |
| bacillus | bacilli |
| bronchus | bronchi |
| focus | focuses or foci |
| fungus | fungi or funguses |
| locus | loci |
| nucleus | nuclei or nucleuses |
| radius | radii or radiuses |
| stimulus | stimuli |
| syllabus | syllabuses or syllabi |
-us to -ra
| Singular | Plural |
| corpus | corpora or corpuses |
| genus | genera or genuses |
-x to -ces
| Singular | Plural |
| apex | apexes or apices |
| appendix | appendixes or appendices |
| codex | codices |
| helix | helices or helixes |
| index | indexes or indices |
| matrix | matrices or matrixes |
| thorax | thoraxes or thoraces |
| vortex | vortices or vortexes |
Other irregular plurals
| Singular | Plural |
| man | men |
| woman | women |
| child | children |
| brother | brothers, religious brethren |
| tooth | teeth |
| foot | feet |
| goose | geese |
| mouse | mice |
| louse | lice |
| ox | oxen |