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]

Moby Thesaurus II by Grady Ward, 1.0:

109 Moby Thesaurus words for "rarity":
   abnormality, airiness, airy texture, amazement, anomaly,
   astonishing thing, astonishment, attenuation, chinchiness,
   chintziness, conversation piece, curio, curiosity, dearth,
   delicacy, diaphanousness, dilutedness, dilution, ethereality,
   exception, exceptionality, exiguity, exility, extraordinariness,
   fabulousness, fewness, find, fineness, flimsiness, frailty,
   gauziness, gazingstock, gossameriness, gracility, impossibility,
   improbability, inconceivability, incredibility, increditability,
   infrequence, infrequency, insubstantiality, laciness, lightness,
   marvel, marvelment, marvelousness, meagerness, miracle,
   miserliness, mistiness, museum piece, mythicalness, niggardliness,
   nonesuch, occasionalness, oddity, paperiness, paucity, phenomenon,
   poverty, prodigiosity, prodigiousness, prodigy, quite a thing,
   rareness, remarkableness, restrictedness, scant sufficiency,
   scantiness, scarceness, scarcity, scrimpiness, seldomness,
   sensation, sight, skimpiness, slenderness, slightness, slimness,
   slowness, smallness, something else, sparseness, sparsity,
   spectacle, stinginess, strange thing, stringency, stunner,
   stupendousness, subtility, tenuity, thinness, tightness, treasure,
   uncommonness, unfrequentness, uniqueness, unordinariness,
   unusualness, unwontedness, vagueness, wateriness, weakness,
   wispiness, wonder, wonderful thing, wonderment