The Collaborative International Dictionary of English v.0.48:

ostracism \os"tra*cism\, n. [Gr. 'ostrakismo`s, fr.
   'ostraki`zein to ostracize. See Ostracize.]
   1. (Gr. Antiq.) Banishment by popular vote, -- a means
      adopted at Athens to rid the city of a person whose talent
      and influence gave umbrage.
      [1913 Webster]

   2. Banishment; exclusion; as, social ostracism.
      [1913 Webster]

            Public envy is as an ostracism, that eclipseth men
            when they grow too great.             --Bacon.
      [1913 Webster]

            Sentenced to a perpetual ostracism from the . . .
            confidence, and honors, and emoluments of his
            country.                              --A. Hamilton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ostracism
    n 1: the state of being banished or ostracized (excluded from
         society by general consent); "the association should get
         rid of its elderly members--not by euthanasia, of course,
         but by Coventry" [syn: banishment, ostracism,
         Coventry]
    2: the act of excluding someone from society by general consent

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "ostracism":
   ban, banishment, blackball, blackballing, blacklist, boycott,
   boycottage, clearance, complaint, defrocking, degradation,
   demotion, depluming, deportation, deprivation, detachment,
   dim view, disagreement, disappointment, disapprobation,
   disapproval, disbarment, discard, discontent, discontentedness,
   discontentment, disenchantment, disesteem, disfavor, disfellowship,
   disgruntlement, disillusion, disillusionment, disjunction,
   displacement, displeasure, displuming, disposal, disposition,
   disrespect, dissatisfaction, dissent, distaste, ejection,
   elimination, eradication, exclusion, excommunication, exile,
   expatriation, expulsion, extradition, fugitation, indignation,
   liquidation, low estimation, low opinion, objection, opposition,
   opposure, ostracization, outlawing, outlawry, proscription,
   protest, purge, rejection, relegation, removal, riddance,
   rustication, severance, stripping, suspension, thumbs-down,
   transportation, unfrocking, unhappiness, withdrawal