The problem with the term 'NTR' is that many people have their own definition of what it exactly is. I'll give you an example to illustrate what you'd expect from something with an NTR theme because it's hard to give a concrete definition.
A traditional NTR story is, at its most basic form, when a girl who has established feelings for her significant other at the beginning of the story, is shown to be RELUCTANT to change that attitude. By the end of the story, the girl has a major change of heart and puts someone else in higher importance than her initial significant other.
NTR isn't limited to SO's as there are lots of works with jealousy elements that are also considered NTR which include childhood friend crush who you've never made a move on being taken by your school bully, etc.
Also, POV matters. If you're playing as the cheater, then it's netori. If you're playing as the girl who gets stolen or the guy getting cucked, then it's NTR.