Eschatology is the study of the last things, or the end times. The Lexham Bible Dictionary says that it includes death, the intermediate state, the afterlife, judgment, the millennium, heaven, and hell, but it also refers to the time of Jesus’ second coming.

• Logos Bible Study App: What Is Eschatology? 4 Views, Why There’s Disagreement & More
• The Gospel Coalition: Views of the Millennium

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The Millennium refers to the 1000-year period of Christ's reign in Revelation 20:3. The four different views deal with when Christ returns.