The Collaborative International Dictionary of English v.0.48:

Waiver \Waiv"er\, n. (Law)
   The act of waiving, or not insisting on, some right, claim,
   or privilege.
   [1913 Webster]

WordNet (r) 3.0 (2006):

waiver
    n 1: a formal written statement of relinquishment [syn:
         release, waiver, discharge]

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "waiver":
   OK, abandonment, abdication, abeyance, abjuration, abjurement,
   abolishment, abolition, abrogation, admission, allowance,
   annulment, cancel, canceling, cancellation, cassation, cessation,
   cession, charter, circumscription, cold storage, concession,
   consent, countermand, counterorder, deed of release, defeasance,
   desistance, discontinuance, dispensation, dropping out, exception,
   exemption, extenuating circumstances, forbearance, forswearing,
   grain of salt, grant, handing over, hedge, hedging, invalidation,
   leave, liberty, license, limitation, mental reservation,
   modification, nonexercise, nullification, okay, patent, permission,
   permission to enter, qualification, quitclaim, recall, recantation,
   release, relinquishment, renege, renouncement, renunciation,
   repeal, rescinding, rescindment, rescission, reservation,
   resignation, restriction, retraction, reversal, revocation, revoke,
   revokement, salvo, setting aside, special case, special permission,
   special treatment, specialness, specification, surrender,
   suspension, ticket, ticket of admission, vacation, vacatur,
   voidance, voiding, vouchsafement, waiving, withdrawal, withdrawing,
   write-off, yielding



Bouvier's Law Dictionary, Revised 6th Ed (1856):

WAIVER., The relinquishment or refusal to accept of a right.
     2. In practice it is required of every one to take advantage of his
rights at a proper time and, neglecting to do so, will be considered as a
waiver. If, for example, a defendant who has been misnamed in the writ and
declaration, pleads over, he cannot afterwards take advantage of the error
by pleading in abatement, for his plea amounts to a waiver.
     3. In seeking for a remedy the party injured may, in some instances,
waive a part of his right, and sue for another; for example, when the
defendant has committed a trespass on the property of the plaintiff, by
taking it away, and afterwards he sells it, the injured party may waive the
trespass, and bring an action of assumpsit for the recovery of the money
thus received by the defendant. 1 Chit. Pl. 90.
     4. In contracts, if, after knowledge of a supposed fraud, surprise or
mistake, a party performs the agreement in part, he will be considered as
having waived the objection. 1 Bro. Parl. Cas. 289.
     5. It is a rule of the civil law, consonant with reason, that any one
may renounce or waive that which has been established in his favor: Regula
est juris antique omnes licentiam habere his quae pro se introducta sunt,
renunciare. Code 2, 3, 29. As to what will amount to a waiver of a
forfeiture, see 1 Conn. R. 79; 7 Conn. R. 45; 1 Jo Cas. 125; 8 Pick. 292; 2
N. H, Rep. 120 163; 14 Wend. 419; 1 Ham. R. 21. Vide Verdict.