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]