An Etymological Dictionary of Astronomy and Astrophysics

1. Math.: An equation that is valid for all values of its variables. A mathematical relationship equating one quantity to another.
   

2. Logic: An assertion that two terms refer to the same thing.
   

3. Psychology: The character of persisting unchanged. The feeling that one knows who one really is.

Etymology (EN): From M.Fr. identité, from L.L. identitas
“sameness,” from ident-, combining form of L. idem “the same; at the same time; also; moreover,” from id “it, that one” + demonstrative suffix -dem + -itas a suffix used to form abstract nouns expressing state or condition.

Etymology (PE): Idâni, from iduni, from Mid.Pers. êdônih “being this, being that, being so, the manner of being,” from êdôn “thus, so,” Mod.Pers. idun “this, in this manner, now;” O.Pers. aita- demonstrative pronoun “this;” Av. aēta- “this; this here; this now,”
aētaδ- (adv.) “here, there; then, thus; thereupon;” cf. Skt. etad “this,” iti “thus, in this manner;” akin to L. idem, as above.