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]

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "secede":
   abandon, agree to differ, agree to disagree, apostacize,
   apostatize, be at variance, be in dissent, beg to differ, betray,
   bolt, break away, change sides, defect, desert, differ, disagree,
   disagree with, discord with, dissent, dissent from, divide on,
   drop out, fall away, fall off, forsake, go over, leave, let down,
   not agree, oppose, pull out, quit, rat, run out on, sell out,
   switch, switch over, take exception, take issue, turn cloak,
   withdraw, withhold assent