Eternity was time without end.
The Q had intended to imprison Q in a comet for eternity. (VOY episode: "Death Wish")
T'Pol thought that if there was a voice of eternity, it would be Eskey Inglesei's. (ENT short story: "Universal Chord")
In 2364, William T. Riker described the disembodied crew of the Sergei G. Gorshkov, who had been kept alive for centuries, as "sharing eternity." (TNG novel: Ghost Ship)
In 2408, Admiral Riker described the Guardian of Forever as "God's window on eternity." (TNG novel: Imzadi)