The Collaborative International Dictionary of English v.0.48:
Millennial \Mil*len"ni*al\, a. Of or pertaining to the millennium, or to a thousand years; as, a millennial period; millennial happiness. [1913 Webster]WordNet (r) 3.0 (2006):
millennial adj 1: relating to a millennium or span of a thousand years [syn: millennial, millennian]