In strict terms, a married man whose partner is cheating on them with another, though it can simply refer to someone who is in a monogamous relationship, where the other partner is cheating on them, irrespective of gender.
From the Old French word cucu.
It first showed up in English around 1250 with the word cokewold,
Wasn't until around 1562 that a female version showed up in English Literature, which was cuckquean.
This is where the one who is a partner of another, who cheats, is called the Cuckold (Cockold) and has been the source of humor, and derision.
In most instances, these partners are the last to know of the infidelity.
Usually the sign of the horn, is used, a reference to the person not knowing about their spouse cheating.
Practice (Associated Acts):
Under normal conditions, the spouse being cheated on, is unaware of the transgression.
Yet as a sexual fetish, the 'cuckold' is not only aware of the infidelity, but is a passive participant.
In this type of scenario, the normally dominant partner becomes submissive, participating only at the direct invitation of their transgressing spouse.
Most instances, the cuckold achieves an enhanced arousal and desire for their cheating spouse.
In China, the color green is symbolic of one who has been cuckold, which is why it is rare to see Chinese Men wearing Green Hats.
Roman Catholic Priests in China have special dispensation, to wear a different colored 'galero'.
The reason is this, is that the translation in China for Cuckold, is literally 'wearing a green hat'.
As a fetish, it is believed that part of the attraction, is that watching their spouse enjoying sex with another, spurs them onto wanting to show their own abilities, to please.
In short a form of competition, resulting from watching their partner having sex with another party.