The word aeon /ˈiːɒn/, also spelled eon (in American and Australian English^[1][2]), 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 (aiwōn) meaning "century". In Greek, it literally refers to the timespan of one hundred years. A cognate Latin word aevum (cf. αἰϝών) for "age" is present in words such as eternal, longevity and mediaeval.^[3]

Although the term aeon may be used in reference to a period of a billion 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.

In astronomy, an aeon is defined as a billion years (10⁹ years, abbreviated AE).^[4]

Roger Penrose uses the word aeon to describe the period between successive and cyclic Big Bangs within the context of conformal cyclic cosmology.^[5]

In Buddhism, an "aeon" or mahakalpa (Sanskrit: महाकल्प) is often said to be 1,334,240,000 years, the life cycle of the world. Yet, these numbers are symbolic, not literal.^[6]

Christianity's idea of "eternal life" comes from the word for life, zōḗ (ζωή), and a form of aión (αἰών)^{[citation needed]}, which could mean life in the next aeon, the Kingdom of God, or Heaven, just as much as immortality, as in John 3:16.^{[unbalanced opinion?]}

According to Christian universalism, the Greek New Testament scriptures use the word aión (αἰών) to mean a long period and the word aiṓnion (αἰώνιον) to mean "during a long period";^[7] thus, there was a time before the aeons, and the aeonian period is finite. After each person's mortal life ends, they are judged worthy of aeonian life or aeonian punishment. That is, after the period of the aeons, all punishment will cease and death is overcome and then God becomes the all in each one (1Cor 15:28). This contrasts with the conventional Christian belief in eternal life and eternal punishment.

Occultists of the Thelema and Ordo Templi Orientis (English: "Order of the Temple of the East") traditions sometimes speak of a "magical Aeon" that may last for perhaps as little as 2,000 years.^[8]

In many Gnostic systems, the various emanations of God, who is also known by such names as the One, the Monad, Aion teleos ("The Broadest Aeon", Greek: αἰών τέλεος), Bythos ("depth or profundity", Greek: βυθός), Proarkhe ("before the beginning", Greek: προαρχή), Arkhe ("the beginning", Greek: ἀρχή), Sophia ("wisdom"), and Christos ("the Anointed One"), are called Aeons. In the different systems these emanations are differently named, classified, and described, but the emanation theory itself is common to all forms of Gnosticism.

In the Basilidian Gnosis they are called sonships (υἱότητες huiotetes; singular: υἱότης huiotes); according to Marcus, they are numbers and sounds; in Valentinianism they form male/female pairs called "syzygies" (Greek συζυγίαι, from σύζυγοι syzygoi).

• Aion (deity)
• Century – Unit of time lasting 100 years
• Kalpa (aeon)
• Millennium – Time period of 1000 years
• Saeculum – comparable Latin concept
• Aeon (company)
• Young's Literal Translation § Eternity or age