aeternus

Aeternus, aeterna, aeternum declines like bonus and is contracted from an older aeviternus, "lasting an age", built on aevum. The adverbial phrase to know is in aeternum, "for ever", with the accusative; ablative aeterno also occurs.

The corpus is heavily ecclesiastical, so it turns up constantly and in fixed phrases: vita aeterna, requiem aeternam, ignis aeternus, Pater aeterne. Worth noticing that even in classical Latin it is a strong word. Perpetuus means unbroken and sempiternus means going on and on; aeternus claims a different order of duration, outside time rather than long inside it, which is precisely why Christian writers seized on it. If a text picks aeternus over perpetuus, that is usually deliberate. English keeps eternal, eternity and sempiternal.