Millennium, in a common Christian belief, the period of 1,000 years before the Last Judgment, during which time Christ and the saints will reign on earth. The word sometimes refers to an ideal society in which there is perfect peace and justice. The Christian belief is not part of the orthodox doctrine but is held by many individual Christians and is prominently proclaimed by certain denominations. The belief is based on the vision described in Revelation 20. In this vision Satan is bound for 1,000 years, and the Christian martyrs are resurrected to reign with Christ. At the end of this period, Christ destroys Satan in a final cataclysmic battle at a place called Armageddon. The battle itself is also called Armageddon.

Some early Christians accepted this vision as a prediction of future events in human history, a doctrine called millennialism, or chiliasm (from a Greek word that means thousand). The Roman Catholic church banished the belief from its body of doctrines but never officially repudiated it. During the Reformation of the 16th century some of the Anabaptists emphasized millennial ideas. In the 17th and 18th centuries the German Pietists of the Lutheran church were millennialists. Adventists, Pentecostals, and certain other groups stress millennialism.

Some Christians, known as premillennialists, believe Christ will return soon, defeat Satan at Armageddon, and inaugurate the conversion of the world and the millennium. Other Christians, known as postmillennialists, believe that the world is already in the millennium and that after a gradual Christian conversion of the world is achieved, Christ will return.