The defendant assigns as error the trial court’s instructions to the jury concerning the issue of his waiver. The trial court instructed the jury that they were to find that there had been a waiver by the defendant if they found that he had intentionally surrendered his visitation rights accorded to him under the separation agreement. The defendant contends that such mere surrender of rights by him would be insufficient to constitute a waiver of the plaintiff’s breach of the contract by denying him his rights to visitation.
“Waiver” is sometimes defined as the intentional and voluntary relinquishment of a known right. Luther v. Luther, 234 N.C. 429, 67 S.E. 2d 345 (1951). This definition is adequate in describing certain types of waivers such as a waiver of the right to have the assistance of counsel at a criminal proceeding or the waiver of the right to trial by jury. However, there must be more than a mere intentional and voluntary relinquishment of a known right in order for there to be a waiver of a right based in contract.
 In order for an alteration to a contract to be enforceable, there must be either an express or implied agreement between the parties that the terms of the contract should be altered. See Klein v. Insurance Co., 289 N.C. 63, 220 S.E. 2d 595 (1975). Generally, an agreement to alter the terms of the contract is treated as any other contract and must be supported by consideration. See Hospital v. Stancil, 263 N.C. 630, 139 S.E. 2d 901 (1965)..
 On the other hand, express or implied agreements not supported by consideration are enforceable in some instances on equitable grounds. The principle of estoppel in pais is one such equitable device which permits an express or implied agreement altering the terms of the contract to be enforced even though not supported by consideration. Estoppel in pais serves as a bar to a party’s assertion that the agreement to alter the terms of a contract lacks consideration:
[W]hen any one, by his acts, representations, or admissions, or by his silence when he ought to speak out, intentionally or through culpable negligence induces another to believe certain facts to exist, and such other rightfully relies and acts on *57such belief, so that he will be prejudiced if the former is permitted to deny the existence of such facts.
Boddie v. Bond, 154 N.C. 359, 365, 70 S.E. 824, 826 (1911). Accord: Matthieu v. Gas Co., 269 N.C. 212, 152 S.E. 2d 336 (1967); In re Covington’s Will, 252 N.C. 546, 114 S.E. 2d 257 (1960); Peek v. Trust Co., 242 N.C. 1, 86 S.E. 2d 745 (1955); Hawkins v. Finance Corp., 238 N.C. 174, 77 S.E. 2d 669 (1953); Long v. Trantham, 226 N.C. 510, 39 S.E. 2d 384 (1946); Thomas v. Conyers, 198 N.C. 229, 151 S.E. 270 (1930); Bank v. Winder, 198 N.C. 18, 150 S.E. 489 (1929). But see Watkins v. Motor Lines, 279 N.C. 132, 181 S.E. 2d 588 (1971).
 In the present case, however, the trial court merely instructed the jury that they were to find a waiver by the defendant of his visitation rights if they found that he intentionally surrendered those rights. Therefore, we find that the trial court’s charge failed to give adequate guidance to the jury with regard to the foregoing principles governing the application of the doctrine of estoppel in pais. The trial court should have instructed the jury that, in order for the jury to find that the defendant had waived his visitation rights under the separation agreement, the jury must first find either that the defendant’s acts, representations or silence induced the plaintiff to believe that the defendant had surrendered his visitation rights and that the plaintiff relied on that belief to her detriment or that the defendant’s express or implied agreement to surrender his visitation rights was supported by consideration. As the charge of the trial court did not adequately explain and apply the foregoing principles, it constituted error prejudicial to the defendant which requires that the defendant be granted a
Judges Martin (Robert M.) and Erwin concur.