According to the OED
, A mounted gun
which is mechanically loaded and fired, delivering a continuous fire of projectile
The machine gun, along with heavy use of artillery, is really what turned World War I into a trench-ridden, body-scattered nightmare. Sure, they had been around in earlier wars, like the Franco-Prussian War, those were still on a small enough scale that the wars could still be fought with old tactics, lightly and quickly.
WWI, however, would not allow it. The problem was, the defense was mechanized, while the offense wasn't - ie, there were no tanks around in the beginning. So you have individual foot soldiers, with rifles, throwing themselves en masse against fortified trenches - armed with machine guns. These things were damn effective, even though you couldn't move them around, and basically mowed down wave after wave of infantry. Infantry was therefore forced to build trenches to protect themselves, resulting in mutually pinned down forces.