The Collaborative International Dictionary of English v.0.48:

Rarity \Rar"i*ty\ (r[a^]r"[i^]*t[y^]; 277), n.; pl. Rarities
   (r[a^]r"[i^]*t[i^]z). [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.
      [1913 Webster]

   2. That which is rare; an uncommon thing; a thing valued for
      its scarcity.
      [1913 Webster]

            I saw three rarities of different kinds, which
            pleased me more than any other shows in the place.
                                                  --Addison.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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