Boethius defined it: eternity is the complete, simultaneous, and perfect possession of endless life. Not time going on forever — time stood up on end.
This distinction matters because many reports of mystical experience describe not a long time but a vertical moment — the present-moment felt as complete, not needing continuation. The Greeks distinguished kairos (right time, opportune time) from chronos (measured time); contemplatives often point at a third register beyond both.