The Collaborative International Dictionary of English v.0.48:

Annulment \An*nul"ment\, n. [Cf. F. annulement.]
   The act of annulling; abolition; invalidation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

annulment
    n 1: the state of being cancelled or annulled [syn:
         revocation, annulment]
    2: (law) a formal termination (of a relationship or a judicial
       proceeding etc) [syn: annulment, invalidation]
    3: the act of abrogating; an official or legal cancellation
       [syn: abrogation, repeal, annulment]

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "annulment":
   abjuration, abjurement, abolishment, abolition, abrogation,
   absolute contradiction, annihilation, broken home, broken marriage,
   cancel, canceling, cancellation, cassation, choking, choking off,
   contradiction, contrary assertion, contravention, controversion,
   counterbalancing, countering, countermand, counterorder, crossing,
   decree of nullity, defeasance, denial, deracination,
   disaffirmation, disallowance, disavowal, disclaimer, disclamation,
   disownment, disproof, dissolution of marriage, divorce,
   divorcement, elimination, eradication, extermination, extinction,
   extinguishment, extirpation, forswearing, frustration, gainsaying,
   grasswidowhood, impugnment, interlocutory decree, invalidation,
   liquidation, negation, neutralization, nullification, offsetting,
   purge, recall, recantation, refutation, renege, renunciation,
   repeal, repudiation, rescinding, rescindment, rescission,
   retractation, retraction, reversal, revocation, revoke, revokement,
   rooting out, separate maintenance, separation, setting aside,
   silencing, snuffing out, stifling, strangulation, suffocation,
   suppression, suspension, thwarting, undoing, uprooting, vacation,
   vacatur, vitiation, voidance, voiding, waiver, waiving, withdrawal,
   write-off