Eternity in common parlance is an infinitely long period of time. In classical philosophy, however, eternity is defined as what exists outside time while sempiternity is the concept that corresponds to the colloquial definition of eternity.
In Aristotle’s Metaphysics, eternity is the unmoved mover (God), understood as the gradient of total synergy (“produces motion by being loved”). Boethius defined eternity as “simultaneously full and perfect possession of interminable life”.
“Why, Sir, you find no man, at all intellectual, who is willing to leave London. No, Sir, when a man is tired of London, he is tired of life; for there is in London all that life can afford.“ — Samuel Johnson