The Collaborative International Dictionary of English v.0.48:

Idiosyncrasy \Id`i*o*syn"cra*sy\, n.; pl. Idiosyncrasies. [Gr.
   ?; 'i`dios proper, peculiar + ? a mixing together, fr. ? to
   mix together; ? with + ? to mix: cf. F. idiosyncrasie. See
   Idiom, and Crasis.]
   A peculiarity of physical or mental constitution or
   temperament; a characteristic belonging to, and
   distinguishing, an individual; characteristic susceptibility;
   idiocrasy; eccentricity.
   [1913 Webster]

         The individual mind . . . takes its tone from the
         idiosyncrasies of the body.              --I. Taylor.
   Idiosyncratic

WordNet (r) 3.0 (2006):

idiosyncrasy
    n 1: a behavioral attribute that is distinctive and peculiar to
         an individual [syn: idiosyncrasy, foible, mannerism]

Moby Thesaurus II by Grady Ward, 1.0:

142 Moby Thesaurus words for "idiosyncrasy":
   aberration, abnormality, animus, anomaly, aptitude, aroma,
   attribute, badge, banner, bent, bias, brand, cachet, cast,
   character, characteristic, conceit, configuration, constitution,
   crackpotism, crank, crankiness, crankism, crotchet, crotchetiness,
   cut, deviancy, deviation, device, diathesis, differentia,
   differential, differentness, disposition, distinctive feature,
   divergence, dottiness, earmark, eccentricity, erraticism,
   erraticness, feature, figure, flavor, freakiness, freakishness,
   grain, gust, habit, hallmark, idiocrasy, image, impress,
   impression, inclination, index, indicant, indicator, individualism,
   insignia, irregularity, keynote, kidney, kink, leaning, lineaments,
   maggot, make, makeup, mannerism, mark, marking, measure,
   mental set, mettle, minauderie, mind, mind-set, mold, nature,
   nonconformity, note, oddity, odor, particularity, peculiar trait,
   peculiarity, picture, predilection, predisposition, preference,
   proclivity, propensity, property, quality, queerness, quip, quirk,
   quirkiness, representation, representative, savor, seal, set,
   shape, sigil, sign, signal, signature, singularity, slant, smack,
   specialty, stamp, strain, strangeness, streak, stripe, sure sign,
   symptom, taint, tang, taste, telltale sign, temper, temperament,
   tendency, token, trademark, trait, trick, trick of behavior, turn,
   turn of mind, twist, type, unconventionality, unnaturalness, warp,
   whim, whimsicality, whimsy