It is indeed something else.
NTR is short for "Netorare", which translates to "stolen away". In essence, it's a fetish about your partner being stolen, often through sexual prowess. So, quite a bit different from just cheating, although cheating is usually a part of it.
There's a variant, "Netori", which is about stealing someone else's partner. It is usually understood that this is not meant when using the NTR abbreviation.
There's also a variation about you being stolen from a partner you love, which I currently don't remember the name of, and probably a dozen others with their respective monikers.
Lately, NTR has come to be used for most content that has the perspective character cuckolded, dumped, cheated on, or anything else that makes the player/reader feel jealous or betrayed.
That's what people want to avoid when checking for NTR, so it's probably the most useful definition, even if it strays a bit from the original meaning.
Hope that clears it up a bit.