What is "first declension"

n. In Latin, a pattern of inflection of a group of nouns that are declined (inflected) in the same way, and which have an ''-a-'' in their stems.

The first declension is a category of declension that consists of mostly feminine nouns in Latin and Ancient Greek with the defining feature of a long ā (analysed as either a part of the stem or a case-ending). In Greek grammar, it is also called the alpha declension, since its forms have the letter α, at least in the plural.

In Latin and Greek grammar, the first declension is analyzed as a thematic declension. But its lack of a Proto-Indo-European thematic vowel (o or e) and of any nominative singular ending (ordinarily -s or -os) doesn't neatly place it within either of the Proto-Indo-European nominal categories, thematic and athematic. Therefore, it is assumed to be a newer formation: a suffix based on the neuter plural ending *-(e)h₂, forming a collective noun.