The Collaborative International Dictionary of English v.0.48:

Proclivity \Pro*cliv"i*ty\, n. [L. proclivitas: cf. F.
   proclivit['e].]
   [1913 Webster]
   1. Inclination; propensity; proneness; tendency. "A
      proclivity to steal." --Abp. Bramhall.
      [1913 Webster]

   2. Readiness; facility; aptitude.
      [1913 Webster]

            He had such a dexterous proclivity as his teachers
            were fain to restrain his forwardness. --Sir H.
                                                  Wotton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

proclivity
    n 1: a natural inclination; "he has a proclivity for
         exaggeration" [syn: proclivity, propensity, leaning]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "proclivity":
   a thing for, affinity, animus, aptitude, aptness, bag, bent, bias,
   cast, character, chosen kind, conatus, conduciveness, constitution,
   cup of tea, delight, diathesis, disposition, druthers, eagerness,
   eccentricity, fancy, fascination, favor, feeling for, grain,
   idiosyncrasy, inclination, inclining, individualism, kidney,
   leaning, liability, liking, make, makeup, mental set, mettle, mind,
   mind-set, mold, mutual affinity, mutual attraction, nature,
   partiality, particular choice, penchant, personal choice,
   predilection, predisposition, preference, prejudice, prepossession,
   probability, proneness, propensity, readiness, sensitivity to, set,
   slant, soft spot, stamp, strain, streak, stripe, style,
   susceptibility, sympathy, taste, temper, temperament, tendency,
   thing, tropism, turn, turn of mind, twist, type, warp, weakness,
   willingness