not widely known; not frequently used or experienced; uncommon or unusual ⇒ a rare word
occurring seldom ⇒ a rare appearance
not widely distributed; not generally occurring ⇒ a rare herb
(of a gas, esp the atmosphere at high altitudes) having a low density; thin; rarefied
uncommonly great; extreme ⇒ kind to a rare degree
exhibiting uncommon excellence; superlatively good or fine ⇒ rare skill
highly valued because of its uncommonness