A buddy of mine who has a masters in world religion and studies ancient Greek says that the word ETERNAL is NOT a word in ancient Greek. Figuring it is used 500+ times in the bible I'd imagine it's in there a few times in greek at least. Could you do me a favor and find out if the word *AIONIOS* is indeed an ancient Greek word.
Actually AIONIOS is only used 71 times in the Greek New Testament and twice in the Septuagint, the Greek translation of the Old Testament.
Please go to http://www.blueletterbible.org/cgi-bin/words.pl?book=Mat&chapter=25&verse=46&strongs=166&page=1&flag_full=1 for the analysis of the word “Aionios” from Thayer’s Greek Lexicon. At that place you can see that the word was used in the Septuagint, which preceded the time of Christ. It was also used in other Greek writings before it was used in the New Testament. This location quotes multiple instances in the writings of Philo of Alexandria (20 BC-50 AD), as well as examples from the Apocrypha.
We use the word eon to designate a long period of time. To the Greeks, however, it meant without a definite beginning or end.