NTR is usually meant these days when someone's love interest/spouse (or perceived as such by the reader) is being taken away, or seduced, by someone else. Be they in an actual relationship or not. That's why the manga of Kodama Naoko is titled as such.
The historical meaning about erogames (instilling a strong emotion of jealousy/despair in the player's mind by making the protagonist's target being fucked by someone else) has been lost along the way.
If taken literally, any resolved love triangle is NTR. So, technically, if Touko chooses one, either Sayaka or Yuu will be "NTR'd"