Much of the modern military's technological efforts, under the rubric of command and control, seek to reduce the fog of war, although the 2003 invasion of Iraq demonstrated that even the most advanced technology does not eliminate it; the coalition military conducted the entire operation as if the Iraqi use of WMD could happen at any moment, although later it was found that no such weapons were available to military units.
In video games, (particularly real time strategy games) it usually specifically means the ability of enemy units to pass through unoccupied or unexplored sections of the map without being seen, and may be available as a switchable option.
The term is usually ascribed to the Prussian military analyst Karl von Clausewitz, who wrote: "The great uncertainty of all data in war is a peculiar difficulty, because all action must, to a certain extent, be planned in a mere twilight, which in addition not unfrequently — like the effect of a fog or moonshine — gives to things exaggerated dimensions and unnatural appearance."