adj. (rarer, rarest) 1 seldom done or found or occurring, uncommon, unusual, few and far between. 2 exceptionally good (had a rare time). 3 of less than the usual density, with only loosely packed substance (the rare atmosphere of the mountain tops). rare bird = RARA AVIS. rare earth 1 a lanthanide element. 2 an oxide of such an element. rare gas = noble gas. rareness n.
[ ME f. L rarus ]