The Collaborative International Dictionary of English v.0.48:

Mannerism \Man"ner*ism\, n. [Cf. F. mani['e]risme.]
   1. Adherence to a peculiar style or manner; a characteristic
      mode of action, bearing, behavior, or treatment of others.
      [1913 Webster]

   2. Adherence to a peculiar style or manner carried to excess,
      especially in literature or art.
      [1913 Webster]

            Mannerism is pardonable,and is sometimes even
            agreeable, when the manner, though vicious, is
            natural . . . . But a mannerism which does not sit
            easy on the mannerist, which has been adopted on
            principle, and which can be sustained only by
            constant effort, is always offensive. --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mannerism
    n 1: a behavioral attribute that is distinctive and peculiar to
         an individual [syn: idiosyncrasy, foible, mannerism]
    2: a deliberate pretense or exaggerated display [syn:
       affectation, mannerism, pose, affectedness]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "mannerism":
   Gongorism, affectation, affectedness, airs, airs and graces, aroma,
   artfulness, artifice, artificiality, attribute, badge, brand,
   cachet, cast, character, characteristic, command of language,
   configuration, cut, differentia, differential, distinctive feature,
   earmark, eccentricity, euphemism, euphuism, exaggeration,
   expression of ideas, facade, false front, false show, fashion,
   feature, feeling for words, feigned belief, figure, flavor,
   form of speech, front, grace of expression, grandiloquence, gust,
   habit, hallmark, hyperelegance, hypocrisy, idiocrasy, idiosyncrasy,
   image, impress, impression, index, individualism, inflation,
   insincerity, keynote, lineaments, literary style, lugs, manner,
   manner of speaking, manneredness, mark, marking, mere show,
   minauderie, mode, mode of expression, mold, nature, oddness, odor,
   overelaboration, overelegance, overniceness, overrefinement,
   particularity, peculiar trait, peculiarity, personal style,
   preciosity, preciousness, pretense, pretension, pretentiousness,
   property, prunes and prisms, public image, purism, put-on,
   putting on airs, quality, queerness, quirk, rhetoric, savor, seal,
   sense of language, sham, shape, show, singularity, smack,
   specialty, stamp, strain, style, stylishness, stylistic analysis,
   stylistics, taint, tang, taste, the grand style, the plain style,
   the sublime, token, trademark, trait, trick, trick of behavior,
   unnaturalness, vein, way