1640s, from Late Latin aeon, from Greek aion "age, vital force; a period of existence, a lifetime, a generation; a long space of time," in plural, "eternity," from PIE root *aiw- "vital force, life, long life, eternity" (cognates: Sanskrit ayu "life," Avestan ayu "age," Latin aevum "space of time, eternity," Gothic aiws "age, eternity," Old Norse ævi "lifetime," German ewig "everlasting," Old English a "ever, always"). Related: Eonian; eonic.