I think the textbook answer is that it is something that a person finds shameful, embarassing or repugnant. I can't say exactly how it pushes my buttons, because as with most aspects of bdsm, it just seems to click. I have used humiliation as a punishment (negative reinforcement) and as an opportunity for my sub to demonstrate submission. I prefer the latter.

Humiliation is an apsect of bdsm that came to me later, after I had discovered at a relatively early age that bondage or restraint of any kind caused a sexual reaction in me. The restraint is a very obvious display of power differential. Humiliation, on the other hand, is much more mental. It can be extremely subtle. In other words, it is not necessarily about a sub having to urinate in her clothes or eat out of a bowl - it can be something as simple as a certain position. A good Domme friend of mine told me not long ago that when she wants to humiliate a sub, she makes him or her send her a photo of the himself/herself on the toilet. Not a scat thing or anything like that - just a picture of the sub sitting naked on the toilet. It is amazing, she told me, how many people will not hesitate to send any kind of picture of themselves she wants, but will dread submitting a shot of themselves on the toilet because of the inherent embrassment.

I look forward to reading the responses you get to this, because there are a broad range of things people find humiliating. Body writing, when certain words are used, is a favorite of mine, especially under th clothes when the sub has to go to work. Forced nudity for prolonged periods are another. I'm sure there are many subs out there who will say "I love that!" But then, the fact that a sub might get aroused by it does not mean it's not humiliating to her, as humiliation and arousal often go hand in hand.