NTR is when your beloved girlfriend, fiancée, or wife is stolen from you, either directly or indirectly.
NTR IS NOT:
1. When your mother or sister is taken away from you, they don't belong to you; they're your family, whether by blood or not. And if you liked them, you're an idiot for staying silent.
2. When a classmate or coworker is taken away from you, she doesn't belong to you either. If you liked her, you're still an idiot for not saying anything.
3. When your childhood friend is taken away from you, the fact that you promised each other that you'd get married as children is no guarantee, even if you do nothing, someone else will have sex with her.