ohh fuck... you will see that different people have different definitions of what is NTR, but essentially, is a plot type when a love interest of the main character ended up having sex/in a relationship with someone else.
while the exact definition is complex (some guys are extremely sensible to the point it has become a joke to call them the NTR police, as pretty much everything is NTR to them), the idea is that this is against MC's wishes, as that would be sharing, not NTR.
But yeah, you kind of opened a a complex question, but the official definition here in f95 is this
[Designed to cause jealousy by having the romantic interest involved with someone other than the MC.]