I think safe touch would be touch that makes neither of the people feel uncomfortable or that harms their relationship in any way.

An example of unsafe touch from my past: when I was an undergrad, one of my professors would touch me whenever I had to go to his office to go over an assignment with him. He would somehow manage to try to put his arm around me as he hovered close to read my paper over my shoulder. Or he would stand next to me so close that our bodies touched. And when I moved away, somehow, he would do it again. This made me very uncomfortable so it was not "safe" for me. Yet, the actual touches he did give me (arm around shoulder, shoulders and arms touching, etc.) were not in and of themselves "unsafe" because if I had received physically identical touches from a friend, boyfriend, or family member, I probably would have considered them "safe." It was the feelings of uncomfortableness, inappropriateness, and unwantedness that made them unsafe to me. This unwanted touch did harm my relationship with this man, because it made me never want to talk to him or go to his office, and my grade suffered because of this.