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.