The Collaborative International Dictionary of English v.0.48:

Nullify \Nul"li*fy\, v. t. [imp. & p. p. Nullified; p. pr. &
   vb. n. Nullifying.] [L. nullificare; nullus none + -ficare
   (in comp.) to make. See Null, a., and -fy.]
   To make void; to render invalid; to deprive of legal force or
   efficacy.
   [1913 Webster]

         Such correspondence would at once nullify the
         conditions of the probationary system.   --I. Taylor.
   [1913 Webster]

   Syn: To abrogate; revoke; annul; repeal; invalidate; cancel.
        See Abolish.
        [1913 Webster]

WordNet (r) 3.0 (2006):

nullify
    v 1: declare invalid; "The contract was annulled"; "void a plea"
         [syn: invalidate, annul, quash, void, avoid,
         nullify] [ant: formalise, formalize, validate]
    2: show to be invalid [syn: invalidate, nullify] [ant:
       validate]
    3: make ineffective by counterbalancing the effect of; "Her
       optimism neutralizes his gloom"; "This action will negate the
       effect of my efforts" [syn: neutralize, neutralise,
       nullify, negate]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "nullify":
   abate, abjure, abolish, abrogate, absolve, annihilate, annul,
   assert the contrary, belie, bring to naught, bring to nothing,
   buffer, cancel, cancel out, come to nothing, compensate, confine,
   contest, contradict, contravene, controvert, counter, counteract,
   counterbalance, countermand, counterorder, countervail, cross,
   declare a moratorium, deny, disaffirm, disallow, disannul, disavow,
   disclaim, disown, disprove, dispute, do away with, forswear,
   frustrate, gainsay, impugn, invalidate, join issue upon, limit,
   make void, negate, negativate, negative, neutralize, not accept,
   not admit, obliterate, offset, oppose, override, overrule, quash,
   recall, recant, refuse to admit, refute, renege, renounce, repeal,
   repudiate, rescind, restrict, retract, reverse, revoke, set aside,
   stultify, suspend, take back, take issue with, thwart, undo,
   vacate, vitiate, void, waive, wipe out, withdraw, write off