An Etymological Dictionary of Astronomy and Astrophysics

1. General: An indefinitely long period of time; an age.
   

2. Geology: A unit of geologic time equal to one billion years; US eon.

Etymology (EN): L. aeon, from Gk. aion “age, eternity;” akin to Av. āiiu- “duration, period, lifetime;” Skt. áyu- “life, longevity.”

Etymology (PE): Âyu, from Av. āiiu- “duration, period, lifetime” (Sogd. āy “life, age”), as above.