non
traditional
realtions
pretty sure thats what msst people mean when they use the term. usually its just used to refrence cheating or cuckoldry but could be pimping, open relations, polyamory, ect.
anything involving others into the situation. not everyones cup of tea but generally where youll find DP and gangbang content beyond harem group stuff which is generally one male multiple female. ntr usually plays out in adding more men into a female characters life instead