I thought at least technically they're synonyms. Do you think I should change the OP?
To quote: "NTR is NeToRare, or in Kanji, [寝取られ], and means 'cheating
with', referring
to the act itself rather than to an individual (the way it was explained to me was that it's the difference between saying 'It's a robbery!' vs. 'It's a robber!'), and it is based off of Netorareru [寝取られる], which is the passive form of Netoru, and can be conjugated further, eg.
kanojo ga yuujin ni netorareru [彼女が友人に寝取られる], which basically translates to, 'The girlfriend is
netoru'd by a friend'. We can see this proven by looking at the root word Netoru [寝取る], which also forms the root for Netori [寝取り] and Netorase [寝取らせ], and means solely, 'to sleep with a girl (or guy) that's already sleeping with someone else', aka cheating. Netori (Could be abbreviated as NTI, but has no formally or informally recognized abbreviation in English as of yet) is the active form of cheating, meaning literally, 'stealing someone else's partner'. Netorase (could be abbreviated as NTS, but has no formally or informally recognized abbreviation in English as of yet) is the causative form of Netoru, meaning letting or forcing someone to netoru someone; this could be something like the storyline of the H-game Euphoria, where rape is forced by threat of execution, to something where a couple goes to a bar, finds someone that they want the wife (or husband; it's equal opportunity) to fuck, and they take them home so the other can watch (and maybe fuck their partner afterwards, depending on the extent of the fetish)."
In short, it's mostly people going, "But it's all just cheating so just label it all NTR" that have caused the massive confusion over the term.