Source : Webster's Revised Unabridged Dictionary (1913)

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.

         Such correspondence would at once nullify the
         conditions of the probationary system.   --I. Taylor.

   Syn: To abrogate; revoke; annul; repeal; invalidate; cancel.
        See {Abolish}.

Source : WordNet®

nullify
     v 1: declare invalid; "The contract was annulled"; "void a plea"
          [syn: {invalidate}, {annul}, {quash}, {void}, {avoid}]
          [ant: {validate}]
     2: show to be invalid [syn: {invalidate}] [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},
         {negate}]
     [also: {nullified}]