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]