The Collaborative International Dictionary of English v.0.48:

Secede \Se"cede"\, v. i. [imp. & p. p. Seceded; p. pr. & vb.
   n. Seceding.] [L. secedere, secessum; pref se- aside +
   cedere to go, move. See Cede.]
   To withdraw from fellowship, communion, or association; to
   separate one's self by a solemn act; to draw off; to retire;
   especially, to withdraw from a political or religious body.
   [1913 Webster]

WordNet (r) 3.0 (2006):

secede
    v 1: withdraw from an organization or communion; "After the
         break up of the Soviet Union, many republics broke away"
         [syn: secede, splinter, break away]