n. The condition of being rare, or not dense, or of occupying, as a corporeal substance, much space with little matter; thinness; tenuity: opposed to density: as, the rarity of a gas.n. The state of being uncommon or of infrequent occurrence; uncommonness; infrequency.n. Something that is rare or uncommon; a thing valued for its scarcity or for its unusual excellence.