The table below contains some classically-formed plurals of Latin nouns that have made their way into English. Some of the English nouns have been substantivized from Latin adjectives, such as alumnus, alumna. Many of these words are Latinizations of Greek works, such as octopus.
Latin customarily organizes its nouns into five declensions (numbered first through fifth) and three genders (masculine, feminine, and neuter). Adjectives are in two declensions:
There is not a declension of adjectives corresponding to the fourth and fifth declensions of nouns.
Because adjectives are often used substantively, there is not a clear line between nouns and adjectives. If a Latin adjective of the first-second declension has become an English noun (example optimum), it is marked as either first or second declension according to its forms as if it were a Latin noun. If of the third declension, no consideration is necessary.
Many proper nouns are included, as they might be used as common nouns. In general, they are captialized. For example, Caesar, although originating as a proper noun, became a term for any of several rulers.
Prudence is required in using these forms. For instance, Gemini is already plural, so that to form a term for people of the astrological sign Gemini, an English-style plural Geminis might be necessary.
As the manuscripts of classical Latin span several centuries, and its authors (as those of every language) had their idiosyncracies, forms other than those below are sometimes attested.
|2 M||agendum||agenda||originally "things to be done", but now meaning "a list of things to be done". A hyper-plural agendae is not classically-formed.|
|3 N||lemma||lemmata||also dilemma-dilemmata, trilemma, etc.|
|3 M||octopus||octopodes||not octopi|
|3 N||onus||onera||English: onerous|
|1 F||opera||operae||of related meaning|
|1 F||propaganda||propagandae||two possible sources of the English noun from a Latin participle|
|5 F||species||species||In ordinary use, the singular is not specie, which however is a legitimate English word. That form comes from a Latin prepositional phrase.|
|2 N||virus||vira||The plural is not attested in ancient Latin, but is a modern invention based on Latin models.|