Aion (from Hellenistic Greek: αἰών, romanized: aión, lit. 'long period of time', [ai̯ˈɔːn]) is a Hellenistic deity associated with time, the orb or circle encompassing the universe, and the zodiac. The "time" which Aion represents is perpetual, unbounded, ritual, and cyclic: The future is a returning version of the past, later called aevum (see Vedic Sanskrit Ṛtú). Philosophically and mythologically, especially in the context of the mysteries, Aion is understood as the ontological personification of eternity : the time of indestructible permanence, which is in essence the duration of the fact of Existence, understood as eternal and immutable.

This kind of time contrasts with empirical, linear, progressive, and historical time that Chronos represented, which divides into past, present, and future.

Aion is thus a god of the cyclic ages, and the cycle of the year and the zodiac. In the latter part of the Classical era he became associated with mystery religions concerned with the afterlife, such as the mysteries of Cybele, the Dionysian mysteries, Orphic religion, and the Mithraic mysteries. In Latin, the concept of the deity may appear as Aeternitas, Anna Perenna, or Saeculum. He is typically in the company of an earth or mother goddess such as Tellus or Cybele, as on the Parabiago plate.

Aion is usually identified as the nude or mostly nude young man within a circle representing the zodiac, symbolic of eternal and cyclical time. Examples include two Roman mosaics from Sentinum (modern–day Sassoferrato) and Hippo Regius in Roman Africa, and the Parabiago plate. But because he represents time as a cycle, he may also be presented as an old man. In the Dionysiaca, Nonnus associates Aion with the Horae and says that he:

The imagery of the twining serpent is connected to the hoop or wheel through the ouroboros, a ring formed by a snake holding the tip of its tail in its mouth. The 4th century CE Latin commentator Servius notes that the image of a snake biting its tail represents the cyclical nature of the year.

In his 5th century work on hieroglyphics, Horapollo makes a further distinction between a serpent that hides its tail under the rest of its body, which represents Aion, and the ouroboros that represents the kosmos, which is the serpent devouring its tail.

Martianus Capella (5th century CE) identified Aion with Cronus (Latin Saturnus), whose name caused him to be theologically conflated with Chronos ("Time"), in the way that the Greek ruler of the underworld Plouton (Pluto) was conflated with Ploutos (Plutus, "Wealth"). Martianus presents Cronus-Aion as the consort of Rhea (Latin Ops) as identified with Physis.

In his highly speculative reconstruction of Mithraic cosmogony, Franz Cumont positioned Aion as Unlimited Time (sometimes represented as Saeculum, Cronus, or Saturn) as the god who emerged from primordial Chaos, and who in turn generated Heaven and Earth. Modern scholars call this deity the 'leonto‑cephaline' figure – a winged, lion-headed, nude male, whose torso is entwined by a serpent. He typically holds a sceptre, keys, and / or a thunderbolt. Nobody knows for sure who he was or what he represented, but aside from the lion-head, depictions of him have Aion's icons; in rare instances, his statue appears in mithrea with the human head, and with the lion-head gone, he is indistinguishable from Aion.