The first declension is a category of mostly feminine nouns in Latin and Ancient Greek with the defining feature of a long ā (either analysed as a part of the stem or 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.
Latin nouns and adjectives 
Latin first declension has only one set of endings for both feminine and masculine nouns.
The very small native masculine group includes only a few occupation nouns and nouns imported from the Greek masculine first declension.
- poēta "poet" ← ποιητής poiētēs "one who composes" poetry
- pīrāta "pirate" ← πειρᾱτής peirātēs "one who makes an attempt on" someone
The same endings are used for the feminine gender of Latin adjectives.
Ancient Greek 
Greek nouns 
Greek first declension has two basic classes of feminine endings and one basic class of masculine endings, distinguished by their original nominative singular: long -ā, short -(y)ă, long -ās. But besides the nominative and accusative singular of feminines, and nominative, genitive, and vocative singular of masculines, forms are the same between subclasses.
In the Attic dialect, an ā-ē split divides each class into two subclasses: nouns with ᾱ and nouns with η. By contrast, other dialects tend to generalize the vowel one way or the other — Ionic has only ē, and Doric and Aeolic have only ā.
Greek adjectives 
The feminine of first- and second-declension adjectives uses the -ā class of the first declension:
- -os, -ā/ē, -on
First- and third-declension adjectives, including participles in -nt-, use the -(y)ă class. Here are examples of this class, which is complex because of sound changes involving the y (see Ancient Greek nouns: short a):
- -us, -eia, -u
- -ās, -aina, -an
- -ōn, -ousa, -on
- -ās, -āsa, -an
- -eis, -eisa, -en