The word aeon , also spelled eon (in American and Australian English), originally meant "life", "vital force" or "being", "generation" or "a period of time", though it tended to be translated as "age" in the sense of "ages", "forever", "timeless" or "for eternity". It is a Latin transliteration from the ancient Greek word ὁ αἰών (ho aion), from the archaic αἰϝών (aiwon) meaning "century". In Greek, it literally refers to the timespan of one hundred years. Its latest meaning is more or less similar to the Sanskrit word kalpacode: san promoted to code: sa and Hebrew word olamcode: heb promoted to code: he . A cognate Latin word aevum or aeuum (cf. αἰϝών) for "age" is present in words such as longevity and mediaeval.

Although the term aeon may be used in reference to a period of a thousand million years (especially in geology, cosmology and astronomy), its more common usage is for any long, indefinite period. Aeon can also refer to the four aeons on the geologic time scale that make up the Earth's history, the Hadean, Archean, Proterozoic, and the current aeon, Phanerozoic.