aevum

Aevum, aevi, n., second declension, and the Latin cousin of Greek aion and of English aye and ever. It covers a stretch of time seen from inside: a lifetime, then an age, then duration without end. The phrase in aevum is simply "for ever", and the ablative aevo does hard work as "in the course of time, with the years".

Do not treat aevum and aetas as interchangeable. Aetas is aevitas worn down, and it leans towards a person's age or a defined historical period; aevum leans towards duration itself. Aeternus is aeviternus contracted, so eternity is literally aevum drawn out. Later Latin also uses aevum for the intermediate duration of angels, neither time nor eternity, which is worth knowing before a scholastic text springs it on you.