1. A teaching of the Christian religion that Christ will return after a long period of time when Christianity has brought about a time of prosperity and peace; in other words, Jesus will return after the millennium.
2. A religious belief that the millennium is an era (not necessarily a literal thousand years) during which Christ will reign over the earth, not from a literal and earthly throne, but through the gradual increase of the Gospel and its power to change lives.

After this gradual Christianization of the world, Christ will return and immediately usher the church into their eternal state after judging the wicked.

This is called postmillennialism because the view is that Christ will return after the millennium.