What is Accord And Satisfaction?

Legal Definition
Accord and satisfaction is a contract law concept about the purchase of the release from a debt obligation. It is one of the methods by which parties to a contract may terminate their agreement. The release is completed by the transfer of valuable consideration that must not be the actual performance of the obligation itself. The accord is the agreement to discharge the obligation and the satisfaction is the legal "consideration" which binds the parties to the agreement. A valid accord does not discharge the prior contract; instead it suspends the right to enforce it in accordance with the terms of the accord contract, in which satisfaction, or performance of the contract will discharge both contracts (the original and the accord). If the creditor breaches the accord, then the debtor will be able to bring up the existence of the accord in order to enjoin any action against him.

If a person is sued over an alleged debt, that person bears the burden of proving the affirmative defense of accord and satisfaction.
-- Wikipedia
Legal Definition
An agreement (accord) between two contracting parties to accept alternate performance to discharge a preexisting duty between them and the subsequent performance (satisfaction) of that agreement. An accord and satisfaction differs from a modification in that a modification immediately discharges a preexisting duty, whereas an accord and satisfaction does not discharge a preexisting duty until the agreed upon, alternate performance occurs.
Illustrative caselaw
See, e.g. Rose Inn of Ithaca, Inc. v. Great American Ins. Co., 75 A.D.3d 737 (N.Y. App. Div. 2010).
Legal Definition
An agreement between two persons, one of whom has a right Of action against the other, that the latter should do or give, and the former accept, something in satisfaction of the right of action different from, and usually less than, what might be legally enforced. When the agreement ls executed, and satisfaction has been made, it ls called "accord and satisfaction." Rogers v. Spokane, 9 Wash. 168, 37 Pac. 300; Davis v. Noaks, 3 J. J. Marsh. (Ky.) 494.

Accord and satisfaction is the substitution of another agreement between the parties in satisfaction of the former one, and an execution of the latter agreement Such ls the definition of this sort of defense, usually given. But a broader application of the doctrine has been made in later times, where one promise or agreement is set up in satisfaction of another. The rule is that an agreement or promlse of the same grade wlll not be held to be in satisfaction of a prior one, unless it has been expressly accepted as such; as, where a new promissory note has been given in lieu of a former one, to have the effect of a satisfaction of the former, it must have been accepted on an express agreement to that effect. Pulliam v. Taylor, 50 Miss. 251; Continental Nat. Bank v. Mc-Geoch, 92 Wis. 286, 66 N. W. 606; Heath v. Vaughn, 11 Colo. App. 384, 53 Pac. 229; Story v. Maclay, 6 Mont. 492, 13 Pac. 198; Swofford Bros. Dry Goods Co. v. Goss, 65 Mo. App. 55; Rogers v. Spokane, 9 Wash. 168, 37 Pac. 300; Heavenrich v. Steele, 57 Minn. 221, 58 N. W. 982.
-- Black's Law Dictionary
Legal Definition
The adjustment of a disagreement as to what is due from one to another, and the payment of the agreed amount. See 62 L. R. A. 760.
-- Ballentine's Law Dictionary