Etymology: < classical Latin and scientific Latin -ae, nominative plural ending of masculine and feminine nouns with singulars ending in -a.In fully naturalized and non-technical words the usual English plural ending -s is substituted for -ae , as in arenas , areas , auroras , hyenas , fuchsias , calceolarias , Floras , Faunas , Julias , Marias , Cleopatras , the two Americas . The plural ending in -ae is normally retained in English only in words which are not naturalized or are found only in technical use, as actiniae (see actinia n.), alae (see ala n.1), laminae (see lamina n.), larvae (see larva n.), nebulae (see nebula n.), striae (see stria n.), although in many of these it varies with the plural in -s ; sometimes this variation patterns with particular senses, such as mathematical formulae , theological formulas (see formula n.). Because Latin borrowings of Greek words with nominative plural in -αι (namely masculine and feminine nouns with singulars ending in -η , -α , -ης , -ας ) regularly had the plural ending -ae , many English words showing plural forms in -ae are ultimately of Greek origin, either having entered English via Latin, or (in a small number of cases) showing substitution of -ae for Greek -αι within English. Compare e.g. agape n., agora n.1, amphora n., amygdala n.
This entry has been updated (OED Third Edition, December 2011; most recently modified version published online March 2021).