1630s, from Modern Latin millennium, from Latin mille "thousand" (see million) + annus "year" (see annual); formed on analogy of biennium, triennium, etc. For vowel change, see biennial. First in English in sense of "1,000-year period of Christ's anticipated rule on Earth" (Revelations xx.1-5). Sense of "any 1,000-year period" first recorded 1711. Meaning "the year 2000" attested from 1970.