“Let me say to Bible students that we must be very careful how we use the word “eternity.” We have fallen into great error in our constant use of that word. There is no word in the whole Book of God corresponding with our “eternal,” which, as commonly used among us, means absolutely without end. The strongest Scripture word used with reference to the existence of God, is—“unto the ages of the ages,” which does not literally mean eternally.”
— Morgan, G. Campbell. God’s Methods with Man. New York: Revell, 1898.
But possibly only “aididios” (and its derivation) but it only occurs twice in the entire Bible.
Also, it is never used to describe the duration of punishment for even Satan’s punishment is described to the “ages (subset) of Ages (Universal Set)” – a very long but unknown limited time.
It’s used once to describe God’s Power and in another instance the chains which hold the “apostate angels”.
As shown above, though this chains were to be “eternal” (according to what their sins deserve), God has been Gracious enough to reduce the sentence to “ages of Ages” (since the duration of punishment for any sinful creature cannot exceed Satan’s sentence).
Note that the Greek word “aidios” (and equivalents = eternal) is never used to describe the punishment of Satan nor men.
So, we understand via God’s unchanging Principle as revealed in the Book of James that “Mercy triumphs over Judgment” even to the “one who has shown no mercy” agrees and allows for this sentence changing from “aidiois” (eternal) to at most “ages of the Ages” (limited, but an unrevealed long time).
Men are only described to be punished up to the next “age” (singular, limited time, unrevealed too) and never even to the “ages” (plural) or “ages of Ages” (plural, subset to Set).
See Matthew 25:46 to learn that the final sentencing of disobedient men is to the “next age” (singular, “age-during punishment”, an adjective used here & not a noun) meaning that these will be punished “during the next age” while the righteous will be having life “during this same next age too” reaping the rewards.
That’s how God is Perfectly Fair in Justice too.