The Collaborative International Dictionary of English v.0.48:

Ostracize \Os"tra*cize\, v. t. [imp. & p. p. Ostracized; p.
   pr. & vb. n. Ostracizing.] [Gr. 'ostraki`zein, fr.
   'o`strakon a tile, a tablet used in voting, a shell; cf.
   'o`streon oyster, 'oste`on bone. Cf. Osseous, Oyster.]
   1. (Gr. Antiq.) To exile by ostracism; to banish by a popular
      vote, as at Athens. --Grote.
      [1913 Webster]

   2. To banish from society, by a general consent; to exclude
      from social, political, or private favor; to exclude from
      conversation or friendship; to shun; as, he was ostracized
      by his former friends. A person may be ostracized by a
      formal vote or by a widespread but informal agreement.
      --Marvell.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

ostracize
    v 1: expel from a community or group [syn: banish, ban,
         ostracize, ostracise, shun, cast out, blackball]
    2: avoid speaking to or dealing with; "Ever since I spoke up, my
       colleagues ostracize me" [syn: ostracize, ostracise]