means simply a thousand years. (Latin, mille annus.) In Rev. xx. 2 it is said that an angel bound Satan a thousand years, and in verse 4 we are told of certain martyrs who will come to life again, and “reign with Christ a thousand years.” “This,” says St. John, “is the first resurrection;” and this is what is meant by the millennium.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894