A person who thrives on and enjoys activities which produce an adrenaline surge in that person. Such activities include cave diving, hang gliding, skydiving, skiing, snowboarding, wakeboarding, rock climbing with minimal equipment, stunt piloting of any vehicle, and other such things. AJ's are a sub-culture that is often viewed with dissaproval by mundanes, normals, and people who don't know how to have fun.

I think I've found a way to be an adrenaline junkie in one's every day life as opposed to doing something outlandish like going to Australia and starting your own tribe with nothing but toothpaste. Things like volunteering to handle the dangerous chemicals in your chemistry lab or pissing off bikers while stuck in traffic all supply an ample amount of adrenaline into one's nervous system, and you didn't even have to put on any lycra!
Of course, one must realize that there is a fine line between being an everyday adrenaline junkie and being a masochist. Adernaline junkies will pour the perchloric acid for their lab partner, while a masochist (albeit an extreme masochism) will spill it on himself to get the pleasure. That sure was an ugly tangent.