Troilism is a name for sexual activity involving 3 people.
In evolutionary biology a cuckold is a male who is unknowingly raising offspring that are not genetically related to him, i.e. their wife fucked another guy behind their back and got pregnant and then pretended it was theirs all along.
In human sexuality a cuckold is a male who is complicit in their partners sexual 'infidelity' or perceived infidelity, i.e. cuckoldry is a fetish whereby a man enjoys watching his partner fuck other dudes.
The partner of a cuckold is generally referred to as a 'hot wife'.
The female version of a cuckold is a cuckquean.
A cuckquean is the female equivalent of a cuckold.
A cuckquean is a woman who derives sexual pleasure from the knowledge of and observation of her partners "infidelity". The cuckquean is fully aware of her partners sexual behaviour, and often encouraging of it, or feigning reluctance.
Cuckoldry is distinct from open relationships/swinging/wife swapping because of the element of (desired) sexual humiliation and the make-believe that there is a deception or betrayal occurring.