In military terms: the process of firing two single shots or short bursts in quick succession at the same target. This idea is most often used in urban warfare and close-quarters combat.
The idea behind the double-tap is that it effectively doubles the volume of effective fire being output over a short distance. The closeness of the two shots allows little to no time for recoil to affect the accuracy of the weapon.
The concept was pioneered by Special Forces soldiers who found their efficacy markedly increased by the practice. Especially when firing a weapon that uses light, less powerful rounds, the chance of killing a target in the opening instants of combat is more then doubled.
Many soldiers found that, when bringing their weapon up to fire from a muzzle-down position, they had a tendency to fire early, which would cause the first shot to go low, either missing the target altogether or hitting in a non-vital area. The second shot, once made a habit, would almost always go into a more important area, which increased the rate of first-encounter kills significantly.
Given the effectiveness and ease of the practice, it has been/is being put into practice by soldiers from all branches of the military.