Nah NTR as in netorare is when MC's love interest is stolen. There is another version of it called Netori where the MC does the stealing. Not that it is relevant in this case but both types get tagged with NTR on this website.
Definition I work with:
Through the actions (or inaction) of the protagonist, or within an agreement with the their partner (wife, fiance, girlfriend), the partner ends up engaging in sexual relations with other people. Typically only the wife/fiance/girlfriend has sex with another (otherwise it is swinging - wherein both partners have sex with different people). Or sharing, wherein the protagonist is willing to have their partner have sex with another as long as they are also involved. Or, swapping, wherein the protagonist will swap their partner, allowing them to have sex with another while they, in turn, are allowed to have sex with the other's partner - yes, it is variation on swinging, although typically reserved for couples swinging together as compared to the swinging where it could be anyone.
In most NTR cases neither partner
wants to cheat, although one or both of them is excited by the prospect of sex with another person - either as the participant or as someone that enjoys watching their partner being pleasured by another (yes, they can be involved depending on which version you are talking about *see above*). This can be something that happens based on an agreement between both partners or it could be where the protagonist convinces the other to have sex with someone for their enjoyment or personal gain, basically 'pimping out' or whoring their partner for their own enjoyment.
Regardless of which form it takes, the partner is having sex with another and both parties may enjoy it or, only one enjoys it.
NTR is the satisfaction of a kink/fetish desire, whereas swinging is a lifestyle, polyamory involves romantic feelings with others over the long term, cheating is completely deceptive and cuckolding is where the partner desires sex with others and the protagonist is powerless to stop it from happening (this CAN be a form of NTR, however it isn't the ONLY form of NTR).
All of that said, I see NTR is a much more complex process than standard cheating or cuckolding, which is why we see all of those tags available for visual novels and games here in the forums. In other words, the NTR tag may apply in addition to or in adjacency to the other commonly used tags (swinging, sharing, swapping, cuckolding, cheating, etc.).
Because NTR isn't as simple as a lot of people here try to make it sound, there will continue to be arguments about whether some of the games here require the NTR tag or not.