 In aversive stimulus is generally thought of as an unpleasant stimulus or noxious stimulus. However, those terms are not as accurate as what we would normally like. Specifically, it is a stimulus change or condition that functions as something that you would want to escape. In other words, creating a situation of negative reinforcement or something that when applied would produce a situation of positive punishment. The stimulus itself has the value of being negatively reinforcing or positive punishing. We assume that it is then aversive by that consequence that happens when the application or removal of that particular stimulus. You might have wondered why the stapler was on the desk. It could be an aversive stimulus. See if I open it up and I do this sort of thing, like that, it would likely be aversive. But I think, no, I almost did staple myself that could have been highly aversive and if it was. I probably wouldn't have done that in the future. In aversive stimulus, I would have avoided that feeling in the future.