Acceptance of performance

What does Acceptance of performance mean in American Law?

The definition of Acceptance of performance in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

Perhaps more accurately put, acceptance of less than performance. When one is legally entitled to some performance and knowingly accepts some other or lesser performance, then one may thereby lose claims arising out of the other party’s failure to perform exactly as obligated, e.g., an acceptance of goods when one knows that the goods delivered are not the goods promised. How knowledgeable the accepting party must be to bring about such loss of the right to demand full and perfect performance, and the conditions under which that loss will turn out to be permanent, are complex questions. Cf estoppel; waiver. Sometimes the questions are explicitly dealt with by statute. See acceptance of goods.