In my relationship, basically cheating is going against what boundaries we have set within our relationship. He and I are both pretty jealous, yet at the same time very lenient, so boundaries are strange with us.
I could try to give a complex list of what is and isn't cheating in our relationship, considering at one point we tried poly. I just define it as going against set and agreed upon boundaries in a relationship with another individual. I think it's all up for interpretation.
What would end my relationship? If he went behind my back for an extended period of time planning on being with another person.
Almost crossing the line? To me, allowing an ex of his to flirt with him continuously even though she knows he is married. That bothers me.