An age, an era, a long period of time — and by extension, eternity itself. Aiōn denotes a complete period with its own character: 'this present age' versus 'the age to come.' The phrase eis tous aiōnas tōn aiōnōn ('unto the ages of the ages') is the strongest Greek expression for everlasting.
The NT operates with a two-age framework: the present evil age (Galatians 1:4) dominated by sin and death, and the coming age inaugurated by Christ's resurrection. Believers live in the overlap — the kingdom has broken in, but the old age persists. This 'already/not yet' tension is fundamental to NT theology. Satan is 'the god of this age' (2 Corinthians 4:4), but Christ reigns 'forever and ever' (eis tous aiōnas tōn aiōnōn).