Definition for our world:
A combination of substances that will quickly ignite creating a rise in pressure that when contained will explode from its container. The explosive nature of gunpowder is used by either having it occur in an area where damage is desired (possibly including live victims), or having a release mechanism that will make use of the pressure (ex. propelling a bullet or pushing a piston).
--- Commentary by TerishD ---
Gunpowder is often discussed for a fantasy environment, but mostly assumed not to be available (see: lecture on fantasy gunpowder). Gunpowder actually brought the end to castles and men in plate, as what protected one from simple weapons became large easily hit targets for projectiles powered by gunpowder. Note that even in the most modern games, the ability of gunpowder and technology in general to duplicate the real world quickly spoils any heroic fun as death quickly spoils any 'fun.'