A substance that acts as an antagonist to the effects of adenosine. Since adenosine has receptors in the CNS, the heart, the lungs, on mast cells and in platelets, amonsts others, adenosine antagonists could possibly have much effect on the body.
In practice, the only adenosine antagonists that are widely used are caffeine and theophylline, which have (relativly) mild effects on the body as a whole.
So basically, the term "adenosine antagonist" is a fancy way of saying "caffeine" if you want to pretend to be a 31337 tweaker.