Someone who gets high on adrenaline and possibly addicted to it. They usually supplement this addiction by doing activities that give them adrenaline rushes such as shoplifting, gambling, skydiving, stock market trading and possibly fighting.
Guy skydiving: "Oh my god this f****** awsome."
Observer: "Wow look at that adrenaline junkie, he just can't get enough of it can he."
Some one who seek out and craves thrilling adventures and/or activities to get an adrenaline rush.
Carey Hart flipped his bike backward because he is such the adrenaline junkie.