Everybody knows what it is, no one knows what it is. The nonspecific response of the body to any demand put upon it.
Stress is the spice of life.
That confusion created when one's mind overrides the body's desire to choke the living shit out of some asshole who desperately needs it. The greatest hazard in life is to risk nothing. In the long run, men only hit what they aim at. The difference between great people and everyone else is that great people create their lives actively, while everyone else is created by their lives, passively waiting to see where life takes them next. The difference between the two is the difference between living fully and just existing.