The term originally referred to the Christian belief that Christ would return, and in this Second Coming, he would establish his thousand year reign (the millennium), which would be followed by the Last judgement of all humanity.

The term is now used in a wider sense to describe a certain form that this belief has taken in Christian sects and movements. Norman Cohn, in his classic book, The Pursuit of the Millennium, describes the following beliefs that millenarian movements profess: Salvation is thought to be collective (that is, to be enjoyed by the faithful as a group); it will be realised on this earth, not in an other-worldly heaven; it will come soon, probably within the lifetime of the believers; it will utterly transform all life on earth to perfection; and it will be miraculous, in that it will be accomplished by supernatural agencies. Millenarian sects and movements flourished at various times in Europe from the eleventh to the seventeenth century. Elements of millenarian beliefs are found in many Christian churches and movements today, and some "New Age" groups profess similar beliefs, often shorn of their Christian jargon.