[mil.len.ni.um] n, pl -nia or -niums [NL, fr. L mille thousand + NL -ennium (as in biennium)] (ca. 1638) 1 a: the thousand years mentioned in Revelation 20 during which holiness is to prevail and Christ is to reign on earth b: a period of great happiness or human perfection
2. a: a period of 1000 years b: a 1000th anniversary or its celebration