cuckold - Not simply a man who's wife/girlfriend cheats on him, but rather a man who actually gets aroused by knowing or seeing her in sexual situations with other men, such as flirting kissing, fondling, or sex.  In particular situations where he is NOT participating but instead marginalized, just observing, or even completely ignored.

A man who willingly encourages his wife to sleep with other people because it brings him pleasure. Cuckolds exist on a spectrum between two extremes. One on end is the masochistic cuckold who enjoys humiliations, degradation, and other demeaning activities at the hands of his wife and her lover. The alpha cuckold lies at the opposite end of the spectrum and does not enjoy any form of humiliation and often has a direct say in who his wife sleeps with and when.

NOTE: the spelling of cuckhold is actually incorrect.  A cockhold is to covet, to take control of people, places and thngs usually for monetary gain and personal pleasure. To gaffle, rob, take or be stingy.