yep i agree...i mean it's very vague and i can't figure out the full extent of NTR...meanwhile people talk and act this is perfectly normal
maybe i'm thinking too much <.<
There's basically 2 general definitions for NTR. One refers to works where the main character being the audience proxy is put into a situation where their love interest is having sex with someone else, which is meant to invoke feelings of jealousy or helplessness. In this case NTR is just a synonym for netorare.
The other refers to any work where the love interest has a significant other, but during the course of the story, she (or he sometimes) falls (body if not soul) for another. In this definition the point of view doesn't matter. The cuck doesn't even really have to show up in the story. This definition includes not just netorare, but netori and netorase too.
Neither of these definitions cover all the cases that are generally considered NTR.
What concerns mom ntr, imo it counts, because it doesn't really matter whether thekid has romantic feelings or not. The reader is supposed to in the kid's place.