A formal, somewhat literary term for an endless span of time, often with a Buddhist or philosophical flavor. In everyday speech, 永遠 (えいえん) is more common.
This suffering will not last for eternity.
After an eternity of time, peace finally arrived.
forevermore; for eternity
eternal recurrence; eternal return
From Middle Chinese 永劫 (MC hjwaeng kuwop), a Buddhist term combining 永 'eternal' and 劫 'kalpa' (an immensely long aeon). The Japanese reading えいごう reflects the go-on pronunciation.