- millennium
- millenniuma period of 1,000 years, is spelt with two ls and two ns. The plural is millenniums (preferred) or occasionally millennia. For the problems of reckoning when a new millennium begins, see the remarks at century. Whatever the arguments put forward on the basis of strict reckoning, the natural celebratory point for a new millennium is at the end of the year 1999, 2999, and so on, and not the year following. The adjective millenarian, meaning ‘relating to the millennium’ or ‘believing in the millennium’ (in the theological sense of Christ's second coming), is paradoxically spelt with only one n, being derived not from Latin annus ‘year’ but from milleni, the distributive form (meaning ‘every thousand’) of mille ‘thousand’.
Modern English usage. 2014.