NTR or Netorare, (jap. 寝取られ, 寝(ne) as in 寝取られ means "sleep", in this case, "the lover's sleeping with someone (cheating)". 取られ(torare) is a noun coming from 取られる(torareru), "have something stolen/robbed".
It is a genre of anime (most commonly found in hentai), with a specific idea at its centre - cheating. It has a (main) story where the protagonist has his/her lover stolen by someone. The *** in an Netorare hentai/NTR ****** will focus more on the couple that is having an affair together, rather than the married or dating couple whose relationship is being violated. Netorare/NTR usually has a female character as the one who is cheating on her husband or boyfriend, but it can also be a male character as well sleeping with other girls. Quite often, Netorare hentai/NTR ****** end in depressed, regretful, and rage-filled characters rather than happy endings.