A cuckold (or cuck) is a married man with a sexually unfaithful wife. In current usage it sometimes refers to non-married couples as well, although the common and traditional meaning is a man who is a victim of his wife's adultery.
There are connotations of helplessness and humiliation attributed to the word: implications that the husband lacks the strength to enforce the fidelity due to a man, and is too weak, too stupid, or henpecked to leave or divorce her. Although historically it was a term in general use, nowadays it is also associated with female domination, fantasy, especially power exchange, sexual humiliation and erotic sexual denial.