perpetuus

Perpetuus, perpetua, perpetuum is a first and second declension adjective, from per plus peto, so the idea underneath is "carrying on right through" rather than "lasting for ever". That is the distinction from aeternus. Aeternus is outside time altogether; perpetuus is continuous within it, which is why a wall, a speech, a fever and a magistracy can all be perpetuus.

Two set phrases carry most of the traffic. In perpetuum, "for ever", is an adverbial accusative and takes no noun. Dictator perpetuus, the title Caesar accepted in 44 BC, is the one that got him killed, since perpetuus made the emergency office permanent. The ablative perpetuo also works as a plain adverb, "continuously". English perpetual and perpetuity come straight across.