Millennium \Mil*len"ni*um\ (m[i^]l*l[e^]n"n[i^]*[u^]m), n. [LL., fr. L. mille a thousand + annus a year. See Mile, and Annual.]

A thousand years; especially, the thousand years mentioned in the twentieth chapter in the twentieth chapter of Revelation, during which holiness is to be triumphant throughout the world. Some believe that, during this period, Christ will reign on earth in person with his saints.

Source: Webster's Revised Unabridged Dictionary (1913)