A cuckold is a husband whose wife has cheated on him. Cuckold is also used as a verb. You can make a cuckold of someone if you have sex with their partner without their knowledge. Or you could be cuckolded if you're partner cheats on you. The Oxford English Dictionary traces the term back to the 13th century and if you read Shakespeare in high school you probably came across the term there. Most of the historical references for this term involve heterosexual relationships, although that may just reflect what was written about. Since infidelity and betrayal knows no gender bounds it seems reasonable to use the term for all kinds of committed relationships if you like (although you may not want to choose the cuckold label at all!)
More recently, there has developed something of a community and genre of erotic story and fantasy role play involving the cuckold. If you search for terms like "cuckold stories" or specifically "cuckold hotwife stories" you'll find a host of audio, video, and text materials detailing sexual encounters where a man discovers his wife having sex with another man. While most of these stories are clearly consensual fantasies, it's hard not to note how many of these stories and videos involve complicated (and sometimes problematic) depictions of race, class, and gender.
Related - Husband Wants to Be a Cuckold
Also Known As: cheating, infidelity
Common Misspellings: cuckhold