Aeon \EE-uhn\ , noun;
1. (in Gnosticism) one of a class of powers or beings conceived as emanating from the Supreme Being and performing various functions in the operations of the universe
According to Etymoline.com:
Aeon originates in the 1640's and comes from Latin aeon, which comes from Greek aion, meaning "age, eternity"
According to the Oxford English Dictionary:
1. An age of the universe, an immeasurable period of time; the whole duration of the world, or of the universe; eternity
2. The personification of an age. In [Platonic Philosophy], A power existing from eternity; and emanation, generation, or phase of the supreme deity, taking part in the creation and government of the universe
3. [Geology] [Usually] eon. The largest division of geological time, composed of several eras
4. [Geology] and [Astronomy] One thousand million years
I love this word. It's sort of strange for the English language because we don't typically have three vowels grouped together, but it's meaning conveys such an immeasurable vastness that you don't mind that the word itself is atypical. As you might imagine, an online search of this word results in a lot of hits - this word is used by corporations, insurance agencies, and so much more - there is even a death metal band named Aeon. I also happen to know, from personal experience, that this word is often used in crossword puzzles.
*Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Sunday, September 26