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]