A form of cheating in which someone close to the protagonist is either willingly or unwillingly seduced and stolen away. This isn't necessarily limited to one's lover, but may also include friends, unrequited love interests, or even relatives.
NTR is intended to arouse feelings of jealousy, abandonment, hurt, and heartache, among other negative emotions. The narrative, dialogue, and such in these works often go the extra mile to incite and reinforce these emotions.
Closely related is netori/netoru (寝取り/寝取る). In this genre, the protagonist instead takes the lover of someone else. Also related is cuckolding, in which seeking out others is actively encouraged, even at the expense of the cuckold.
Posts on Danbooru should include the presence of the cheated on in some form, physical or otherwise (such as on the phone, as is common), or mentioned at some significant length.