Some see it as that, yeah. But in reality, it's not. It's when the MC forces a girl, who's already in a relationship, to have sex with him. Some also count when an LI is raped by someone else.
It's often confused with netorase, that is when the MC makes a LI have sex with someone else, for his own enjoyment.
Often, people just take them both together with cheating and a potential LI having sex with anybody else, and put it all under NTR. Some go so far as putting any girl having a boyfriend or ever having sex, also as NTR.
These terms have become so intertwined and mishandled over the years that they've practically merged into one here in the West and as a result, NTR covers pretty much anything in regards to your partner sleeping with others, whether that be with permission or without.
the jist is this though
Netorare - is when the MC's partner is "stolen" from them, usually, there is some sort of blackmail involved and the lover is reluctant at the start. It gets a bit of Stockholm syndrome and the partner is seduced and eventually accepts she likes the other guy better. its supposed to invoke a sense of loss, anguish, jealousy, betrayal
Netorase - this is the concesual one. think swinging, or hot wifing and the likes.
Netori - this is when the MC himself/herself actively seeks to sleep with someone already in a relationship
Ultimately they all involve someone sleeping with someone who isn't their partner, but the actual definitions make them very different
