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]