I won't offer interpretations, only definitions.
If you plug in the word "eternity" into a Strong's Concordance you get the word 'ad in ancient Hebrew, which is first used in Isaiah 57:15 - "For thus saith the high and lofty One that inhabiteth eternity, whose name [is] Holy; I dwell in the high and holy [place], with him also [that is] of a contrite and humble spirit, to revive the spirit of the humble, and to revive the heart of the contrite ones."
In that respect, it is the first time eternity is mentioned as being "forever" as we think of it.
However, the word eternal is also used in Genesis, but when properly translated back to the ancient Hebrew, it means "from the far east" or "to the far east" which was, as it was explained to me, as far as the people of that time can imagine going in this world without falling off lol.