eon

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An indefinitely long period of time; an age.
2. n. The longest division of geologic time, containing two or more eras.

Century Dictionary and Cyclopedia

1. n. A long space of time; a secular period, either indefinite or limited to the duration of something, as a dispensation or the universe: used as equivalent to age, era, or cycle, and sometimes to eternity.
2. n. In Platonic philosophy, a virtue, attribute, or perfection existing throughout eternity. The Platonists represented the Deity as an assemblage of eons. The Gnostics considered eons as certain substantial powers or divine natures emanating from the Supreme Deity, and performing various parts in the operations of the universe.
3. n. The largest divisionof geologic-time: used by J. D. Dana especially in dividing the archœan into astral and archæozoic eons.

Wiktionary

1. n. US Eternity.
2. n. A period of 1,000,000,000 years.
3. n. geology The longest time period used in geology.
4. n. US, informal, hyperbolic A long period of time.
5. n. Gnosticism A spirit being emanating from the Godhead.

GNU Webster's 1913

1. n. An immeasurable or infinite space of time; eternity; a long space of time; an age.
2. n. (Gnostic Philos.) One of the embodiments of the divine attributes of the Eternal Being.

WordNet 3.0

1. n. (Gnosticism) a divine power or nature emanating from the Supreme Being and playing various roles in the operation of the universe
2. n. the longest division of geological time
3. n. an immeasurably long period of time