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:

• the "first-second" declension; this is one declension despite the name. It resembles the first and second declensions of nouns.
• the third declension. It resembles the third declension of nouns.

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.

masculine-feminine pairs

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.

declension, gender	singular	plural	notes
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 F	appendix	appendices
3 M	Apollo	Apollines
2 M	Aquarius	Aquarii
3 M	Aries	Arietes
2 N	bacterium	bacteria
3 M	Caesar	Caesares
2 M	Cancer	Cancri
2 M	Capricornus	Capricorni
3 M	color	colores
3 N	corpus	corpora
2 N	criterion	criteria
2 N	datum	data
3 N	diploma	diplomata
3 M	divisor	divisores
4 F	echo	echus
3 N	enigma	enigmata
2 N	forum	fora
2 M	fungus	fungi
2 M	Geminus	Gemini
3 M	genus	genera
2 M	genius	genii
3 M	honor	honores
3 M	index	indices
2 M	integer	integri
3 M	janitor	janitores
3 N	lemma	lemmata	also dilemma-dilemmata, trilemma, etc.
3 M	Leo	Leones
1 F	Libra	Librae
2 M	Lucifer	Luciferi
3 F	matrix	matrices
3 M	octopus	octopodes	not octopi
3 N	onus	onera	English: onerous
1 F	opera	operae	of related meaning
3 N	opus	opera
2 N	optimum	optima
2 N	phenomenon	phenomena
3 M	Piscis	Pisces
1 F	propaganda	propagandae	two possible sources of the English noun from a Latin participle
2 N	propagandum	propaganda
2 N	quantum	quanta
5 F	rabies	rabies
2 M	radius	radii
2 N	rostrum	rostra
2 M	Sagittarius	Sagittarii
3 M	Scorpio	Scorpiones
5 F	series	series
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.
3 M	spectator	spectatores
3 F	sphinx	sphinges
3 N	stigma	stigmata
2 N	stratum	strata
2 M	Taurus	Tauri
3 F	Venus	Veneres
3 F	Virgo	Virgines
3 M	vertex	vertices
3 M	vortex	vortices
2 N	virus	vira	The plural is not attested in ancient Latin, but is a modern invention based on Latin models.
2 M	Zeus	Zei