Lifetime, life.
Generation, race.
An undefined, relatively long period of time - an eon, an epoch, an age.
An undefined, relatively long period of time - as countable things: an eternity, a perpetuity.
Forever [nominal sense, for adverbial sense see ἀεί]
Remote past.
The current world, current age.
World, space.
Very long stretch of time ahead.
A world to come, a future age.
In Judaism and Christianity: Messianic age.