Without end, and outside the reach of time. Of what does not depend on lasting.
Latin aeternus, from aevum (age, an eternity, a lifetime); the adjective travelled from naming one bounded span of time to naming the absence of any span at all, the outside of time rather than a very long stretch of it.
Its root is a lifetime, not a boundlessness: aevum named an age, one span among others, before the word was stretched to cover every span there is. What is eternal keeps that irony inside it, a word for a single life doing the work of naming what no life can hold.