The name given to the part of the brain that was stimulated in those lab rats mentioned in Dman's writeup in Orgasm as an evolutionary development. Electrical stimulation of the pleasure center caused the rats to choose further stimulation over food, water and just about everything else.
Strangely enough, when volunteer humans had the physiologically similar part of their brain electrically stimulated, the best result researchers could elicit was a "mild buzz" but nothing of the sort of frenzy of pleasure that the rats went through.
i) the human brain doesn't quite work like a rat's (duh)
ii) the human perception of pleasure isn't simple enough to be localised in just one part of the brain
iii) god help us if we discover (and ultimately control) the pleasure pathway in humans...