1.  bimillenary(n = noun.time) bimillennium - the 2000th anniversary (or the celebration of it);
is a kind of anniversary, day of remembrance

2.  bimillenary(n = noun.time) bimillennium - a span of 2000 years;
is a kind of period, period of time, time period

   a span of 2000 years. [WordNet 1.5]

bimillenary, adj. & n.
--adj. of or relating to a two-thousandth anniversary.
--n. (pl. -ies) a bimillenary year or festival.