A belief that the thousand-year reign of Christ described in Revelation 20 is not a future event, but a symbol. The “thousand years” is not understood literally, but figuratively. It is a way of describing the entire period between Christ's first coming (his life, death, and resurrection) and his second coming (the end times). During this time, Christ is the spiritual ruler over the church.
Amillennialism
From Tyndale Bible Dictionary, adapted by Mission Mutual. CC BY-SA 4.0.