Boethius defines eternity thus, "the simultaneously-whole and perfect possession of interminable life." Aquinas poses an interesting objection to this definition:
eternity signifies a certain kind of duration. But duration regards existence rather than life. Therefore the word "life" ought not to come into the definition of eternity; but rather the word "existence."(Summa 1.10.1)God, to whom alone it belongs to be eternal (Summa 1.10.3), doesn't just exist as a cosmic force, He lives eternally. We may talk about other beings having eternal life, but their life is in and through the Holy Trinity who is life and life eternal(Jn 6:51, 14:6). Aquinas replies:
What is truly eternal, is not only being, but also living; and life extends to operation, which is not true of being. Now the protraction of duration seems to belong to operation rather than to being; hence time is the numbering of movement.