The Collaborative International Dictionary of English v.0.48:

Supersede \Su`per*sede"\, v. t. [imp. & p. p. Superseded; p.
   pr. & vb. n. Superseding.] [L. supersedere, supersessum, to
   sit above, be superior to, forbear, omit; super above +
   sedere to sit: cf. F. supers['e]der. See Sit, and cf.
   Surcease.]
   1. To come, or be placed, in the room of; to replace.
      [1913 Webster]

   2. To displace, or set aside, and put another in place of;
      as, to supersede an officer.
      [1913 Webster]

   3. To make void, inefficacious, or useless, by superior
      power, or by coming in the place of; to set aside; to
      render unnecessary; to suspend; to stay.
      [1913 Webster]

            Nothing is supposed that can supersede the known
            laws of natural motion.               --Bentley.
      [1913 Webster]

   4. (Old Law) To omit; to forbear.
      [1913 Webster]

WordNet (r) 3.0 (2006):

supersede
    v 1: take the place or move into the position of; "Smith
         replaced Miller as CEO after Miller left"; "the computer
         has supplanted the slide rule"; "Mary replaced Susan as the
         team's captain and the highest-ranked player in the school"
         [syn: supplant, replace, supersede, supervene upon,
         supercede]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "supersede":
   abandon, act for, change places with, crowd out, cut out, desert,
   discard, displace, double for, fill in for, forsake, ghost,
   ghostwrite, oust, pinch-hit, reject, relieve, replace, represent,
   repudiate, spell, spell off, stand in for, subrogate,
   substitute for, succeed, supplant, swap places with,
   understudy for