The Collaborative International Dictionary of English v.0.48:

aeon \aeon\, AEon \[AE]"on\, n.
   1. A period of immeasurable duration. See Eon.
      [1913 Webster]

   2. (Gnosticism) a divine power or nature emanating from the
      Supreme Being and playing various roles in the operation
      of the universe.

   Syn: eon
        [WordNet 1.5]

   3. the longest division of geological time; two or more eras.

   Syn: eon
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Eon \E"on\, aeon \[ae]"on\, n. [L. aeon, fr. Gr. a'iwn space or
   period of time, lifetime, age; akin to L. aevum. See Age.]
   1. An immeasurable or infinite space of time; eternity; a
      long space of time; an age.
      [1913 Webster]

            The eons of geological time.          --Huxley.
      [1913 Webster]

   2. (Gnostic Philos.) One of the embodiments of the divine
      attributes of the Eternal Being.
      [1913 Webster]

            Among the higher [ae]ons are Mind, Reason, Power,
            Truth, and Life.                      --Am. Cyc.
      [1913 Webster]

   Note: Eons were considered to be emanations sent forth by God
         from the depths of His grand solitude to fulfill
         various functions in the material and spiritual
         universe.
         [1913 Webster]

WordNet (r) 3.0 (2006):

aeon
    n 1: (Gnosticism) a divine power or nature emanating from the
         Supreme Being and playing various roles in the operation of
         the universe [syn: eon, aeon]
    2: the longest division of geological time [syn: eon, aeon]
    3: an immeasurably long period of time; "oh, that happened eons
       ago" [syn: eon, aeon]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "aeon":
   Platonic year, age, ages, annus magnus, blue moon, century, cycle,
   cycle of indiction, date, day, eternity, florid, generation,
   great year, indiction, long, long time, long while,
   month of Sundays, right smart spell, time, years, years on end