Source : Webster's Revised Unabridged Dictionary (1913)

Rarity \Rar"i*ty\, n.; pl. {Rarities}. [L. raritas: cf. F.
   raret['e]. See {Rare}.]
   1. The quality or state of being rare; rareness; thinness;
      as, the rarity (contrasted with the density) of gases.

Source : WordNet®

rarity
     n 1: noteworthy scarcity [syn: {rareness}, {infrequency}]
     2: a rarified quality; "the tenuity of the upper atmosphere"
        [syn: {tenuity}, {low density}]
     3: something unusual -- perhaps worthy of collecting [syn: {curio},
         {curiosity}, {oddity}, {oddment}, {peculiarity}]