A cuckold is a man whose wife has slept with another man (he is said to have made the cuckold "wear the horns").
The term is misused all the time in clip descriptions which refer to the woman. The man is the cuckold, not the woman.
In pornography, the term "cuckold" has taken on a more nuanced/narrow meaning, and is generally applied to relationships where the husband is submissive not only to his wife, but also his wife's lover(s), and often, the couple is white and the lover(s) are black. Further, there is often humiliation of the husband by his wife and her lover(s), and some of the humiliation is with respect to the differential penis sizes (BBC v., er, SWC?).
Look at the etymology, but "cuckold" is at least from the Medieval period (it's in Chaucer).
