Irregular -en Plurals Ablaut Umlaut Plurals
encyclopedia rarely encyclopædia - encyclopedias encyclopediae is rare
Final ex or ix becomes -ices pronounced ɨsiːz, or just adds -es: index - indices
or indexes matrix - matrices
vertex - vertices
Final is becomes es pronounced iːz: axis - axes
crisis - crises testis - testes
Axes, the plural of axis, is pronounced differently from axes æksɨz,
the plural of axe.
Final -ies remains unchanged: series - series
species - species
Final on becomes -a: automaton - automata
criterion - criteria Phenomenon - phenomena more below
polyhedron - polyhedra
Final um becomes -a, or just adds -s: Table 2.2
Nouns in final um become -a, or just adds –s.
addendum Addenda
agendum agenda means a list of items of business at a meeting and
has the plural agendas datum
data Now usually treated as a singular mass noun in both
informal and educated usage, but usage in scientific publications shows a strong UKUS divide. U.S. usage
prefers treating data in the singular in all contexts, including serious and academic publishing. UK usage now
widely accepts treating data as singular in standard English, including educated everyday usage at least in
non-scientific use. forum
foraforums medium
media in communications and computers; now often treated as a singular mass noun mediums spiritualists, or
items of medium size etc. memorandum memorandamemorandums
millennium Millennia
Final us becomes -i second declension, [aɪ] or -era or -ora third
declension, or just adds -es especially in fourth declension, where it would otherwise be the same as the singular:
alumnus- Alumni corpus - Corpora
census - Censuses focus - Foci
genus - genera prospectus - prospecti plural prospectus is rare although technically
correct radius - Radii
syllabus - Syllabi viscus - Viscera
Virus had no plural ending in Latin; the plural in English is usually viruses.