Source : Webster's Revised Unabridged Dictionary (1913)

Nullification \Nul`li*fi*ca"tion\, n. [L. nullificatio contempt.
   See {Nullify}.]
   The act of nullifying; a rendering void and of no effect, or
   of no legal effect.

   {Right of nullification} (U. S. Hist.), the right claimed in
      behalf of a State to nullify or make void, by its
      sovereign act or decree, an enactment of the general
      government which it deems unconstitutional.

Source : WordNet®

nullification
     n 1: the states'-rights doctrine that a state can refuse to
          recognize or to enforce a federal law passed by the
          United States Congress
     2: the act of nullifying; making null and void; counteracting
        or overriding the effect or force of something [syn: {override}]