Not To Be Confused With
Pre-, a-, and post-millennialism differ over the timing and nature of Christ's return relative to the “millennium” of Revelation 20. These are debated, in-house views on eschatology, not tests of orthodoxy.
Premillennialism reads Revelation 20's thousand years as a future earthly reign that Christ inaugurates at his return; dispensational and historic forms differ. It has deep roots in some early fathers and is widespread in modern evangelicalism.