The Collaborative International Dictionary of English v.0.48:

Secession \Se*ces"sion\ (s[-e]*s[e^]sh"[u^]n), n. [L. secessio:
   cf. F. s['e]cession. See Secede.]
   1. The act of seceding; separation from fellowship or
      association with others, as in a religious or political
      organization; withdrawal.
      [1913 Webster]

   2. (U.S. Hist.) The withdrawal of a State from the national
      Union.
      [1913 Webster]

   Secession Church (in Scotland). See Seceder.
      [1913 Webster]

WordNet (r) 3.0 (2006):

secession
    n 1: an Austrian school of art and architecture parallel to the
         French art nouveau in the 1890s [syn: secession,
         sezession]
    2: the withdrawal of eleven southern states from the Union in
       1860 which precipitated the American Civil War
    3: formal separation from an alliance or federation [syn:
       secession, withdrawal]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "secession":
   agreement to disagree, alienation, apostasy, backsliding, betrayal,
   bolt, break, breakaway, breaking, counter-culture, defection,
   dereliction, deserter, desertion, difference, disaccord,
   disagreement, disapprobation, disapproval, disloyalty, disparity,
   dissatisfaction, dissension, dissent, dissentience, dissidence,
   diversity, dropping out, faithlessness, going over,
   minority opinion, nonagreement, nonassent, nonconcurrence,
   nonconformity, nonconsent, opposition, ratting, recidivation,
   recidivism, recreancy, recusance, recusancy, rejection,
   repudiation, retirement, schism, separation, treason,
   turning traitor, underground, variance, walkout, withdrawal