1. I don't know a good definition I just mostly wanted to answer the second question.
2. I have experienced, I'm not sure how else to describe this, stress induced stomach... aches? It's more of a hot/cold feeling than pain, though. I only get it in competitive non-team based multiplayer games. Think of fighting games, such as Street Fighter. I can take about an hour of play-time before I have to stop. But the feeling can last about an hour after that and it's intense enough that I can't really do anything until the feeling subsides. Which sucks because I like playing Street Fighter but my body can't take the stress.