Netorare (寝取られ, lit. "cuckold", also commonly abbreviated as NTR) is a notoriously controversial genre of Hentai (or, in rarer cases, Ecchi), with a degree of infamy perhaps rivaled only by full-blown Guro.
Typically, a heroine will be introduced as having a significant other, which may be a husband, a boyfriend, or even a partner in a BST affair. The story will then show the aforementioned heroine being intimate with another man (another girl just doesn't pack the same punch), thus provoking jealousy in the audience by proxy. The nature of the affair varies, and the parties involved may be willing or unwilling. Sometimes the girl is simply cheating, sometimes it's in the form of the Scarpia Ultimatum, sometimes or the man may simply be pimping his sweetheart out to earn their bread. As you may expect, this is on the bad side of Good Adultery, Bad Adultery.
A fair portion of these works has the cuckolding being done right in front of the established partner, usually with a "why couldn't you do this to/for me"-type verbal/emotional slap in the face.
See also Defiled Forever, Did Not Get the Girl, I Have You Now, My Pretty, Please, I Will Do Anything!, Sacred First Kiss and Scarpia Ultimatum. Very often features an Idiot Plot, and is perhaps one of the few non-comedic techniques which may execute it to good effect. The reverse of netorare is netori, cuckolding someone else's love.