Noun
millionennium (plural millionennia or millionenniums)
Synonym of mega-annum: a period of time consisting of one million years.
Coordinate terms: annum, biennium, triennium, quadrennium, quinquennium, sexennium, septennium, octennium, novennium, decennium, centennium, quincentennium, millennium, decamillennium, centimillennium