Within Christianity, the term "Millennialist"refers to one who believes in the doctrine of the Millennium, in one form or another. The Millennium -- which is derived from the Latin "mille annum" -- refers to a 1000-year period which is associated with Christ and the Kingdom of God. Exactly when this period will occur, and what events will transpire when the Millennium arrives, depends upon one's personal theology and understanding of the Scriptures.
For more information concerning this topic, please consider reading such articles as
From Armageddon To The New Earth. For other views concerning the Millennium, consider looking at the following topics as well:
Amillennial
Millennium
Postmillennial
Premillennial