historic, historical / already-happened, past / former, old, traditional

historian

study of history

(Internet slang, especially anime and manga fandom) events or things from the past which one wants to whitewash/erase from history (due to it being embarrassing, poorly received, etc.); dark past

oral history

historicism

cause-and-effect relationship; causality

consanguinuity

love triangle, eternal triangle / triangle relation, three-cornered relationship

relative pronoun