The Collaborative International Dictionary of English v.0.48:

Invalidate \In*val"i*date\, v. t. [imp. & p. p. Invalidated;
   p. pr. & vb. n. Invalidating.] [From Invalid null.]
   To render invalid; to weaken or lessen the force of; to
   destroy the authority of; to render of no force or effect; to
   overthrow; as, to invalidate an agreement or argument.
   [1913 Webster]

WordNet (r) 3.0 (2006):

invalidate
    v 1: declare invalid; "The contract was annulled"; "void a plea"
         [syn: invalidate, annul, quash, void, avoid,
         nullify] [ant: formalise, formalize, validate]
    2: make invalid for use; "cancel cheques or tickets" [syn:
       cancel, invalidate]
    3: show to be invalid [syn: invalidate, nullify] [ant:
       validate]
    4: take away the legal force of or render ineffective;
       "invalidate a contract" [syn: invalidate, void,
       vitiate] [ant: validate]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "invalidate":
   abate, abolish, abrogate, annihilate, annul, belie, blow sky-high,
   blow up, bring to naught, bring to nothing, buffer, cancel,
   cancel out, come to nothing, counterbalance, countermand,
   counterorder, deflate, disannul, disconfirm, discredit, disprove,
   disqualify, do away with, explode, expose, frustrate, make void,
   negate, negativate, negative, neutralize, nullify, offset,
   override, overrule, prove the contrary, puncture, quash, recall,
   recant, renege, repeal, rescind, retract, reverse, revoke,
   set aside, show up, stultify, suspend, thwart, undercut, undo,
   unfit, vacate, vitiate, void, waive, withdraw, write off