The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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:
       nullification, override]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "nullification":
   abjuration, abjurement, abolishment, abolition, abrogation,
   absolute contradiction, annihilation, annulment, autarky,
   bimetallism, cancel, canceling, cancellation, cassation, choking,
   choking off, contradiction, contrary assertion, contravention,
   controversion, counterbalancing, countering, countermand,
   counterorder, crossing, defeasance, denial, deracination,
   disaffirmation, disallowance, disavowal, disclaimer, disclamation,
   disownment, disproof, economic self-sufficiency, elimination,
   eradication, extermination, extinction, extinguishment,
   extirpation, forswearing, free enterprise, free trade, frustration,
   gainsaying, governmentalism, impugnment, invalidation,
   laissez-faire, laissez-faireism, line, liquidation, localism,
   managed currency, negation, neutralization, noninterference,
   nonintervention, offsetting, party line, party principle,
   planned economy, policy, polity, position, price supports,
   protection, protectionism, public policy, pump-priming, purge,
   recall, recantation, refutation, renege, renunciation, repeal,
   repudiation, rescinding, rescindment, rescission, retractation,
   retraction, reversal, revocation, revoke, revokement, rooting out,
   sectionalism, setting aside, silencing, snuffing out, stifling,
   strangulation, suffocation, suppression, suspension, thwarting,
   undoing, uprooting, vacation, vacatur, vitiation, voidance,
   voiding, waiver, waiving, withdrawal, write-off