NTR for Netori which is when you steal another person's lover.
NTR is not short for netori at all. It stands specifically for netorare. It's not that uncommon for sites to take liberties when it comes to definitions though. e-hentai for an example uses the netorare tag for both netori and netorare and to some point netorase as well.
F95 however uses NTR for netorare and to some extent netorase, but not netori.
F95's specific tag definition is:
"NTR [Designed to cause jealousy by having the romantic interest involved with someone other than the MC.]"
.. as you can see this might involve netorase in many cases, but not netori at all.