...ntr is commonly known as netorare which basically consists of cheating on mc (the mc usually does not realize it or does it too late), but they exist 2 other sub-genres of this for which we do not have a tag, which would be netori and netorase, netori happens when it is the mc who cheats on others, that is, he steals the girls (although the netori tag does not exist here the "cheating" is usually used as a replacement) and finally there is the netorase that happens when the MC is aware that others are cheating on him and actively seeks this to happen.
Personally, I don't like netorare and netorase but I like netori, and if I have to be guided by the comments...
So, just to make it clear here.
NTR means netorare = Japanese for cuckold = being cheated on by your partner, without you being an active part of the cheating (you may be made to watch). There is no other way of defining NTR.
Netorase means willing cuck = applauding your partner having sex with someone else. No abbreviation.
Netori means that you're the one stealing someone else's partner and having a relationship with this person. Also no abbreviation.
In short, NTR only refers to Netorare. This is where many people get it wrong.
F95Zone also define NTR as:
Which means that F95Zone is actually on par with the definition: That NTR means the MC is being cheated on.
The right thing for F95Zone to do would be to create tags for Netorase and Netori, too.