A cuckold is the husband of an adulterous wife. In evolutionary biology, the term is also applied to males who are unwittingly investing parental effort in offspring that are not genetically their own.

The word "cuckold" derives from the cuckoo bird, alluding to its habit of laying its eggs in other birds' nests. The association is common in medieval folklore, literature, and iconography.

English usage first appears about 1250 in the satirical and polemical poem "The Owl and the Nightingale" (l. 1544). The term was clearly regarded as embarrassingly direct, as evident in John Lydgate's "Fall of Princes" (c. 1440). In the late 14th century, the term also appeared in Geoffrey Chaucer's "The Miller's Tale". Shakespeare's poetry often referred to cuckolds, with several of his characters suspecting they had become one.

One often-overlooked subtlety of the word is that it implies that the husband is deceived, that he is unaware of his wife's unfaithfulness and may not know until the arrival or growth of a child plainly not his (as with cuckoo birds).

The female equivalent "cuckquean" first appears in English literature in 1562, adding a female suffix to the "cuck".

A related word, first appearing in 1520, is "wittol", which substitutes "wit" (in the sense of knowing) for the first part of the word, referring to a man aware of and reconciled to his wife's infidelity.