A flashbang is a non-lethal grenade used to thwart the hearing and seeing of enemy combatants. It is especially effective in small corridors and rooms, where anyone in a particular room will be affected by the loud bang and flash (hence the name) created by the flashbang. This device is common among tactical FPS games, where an unobstructed enemy would be otherwise extremely difficult to neutralize.
In some games as a last resort you may be able to kill an enemy by impact or in layman's terms hitting your opponent with the flashbang killing them on impact before the flashbang explodes, the most notable games which evitable proof of the feature exists are Counter-Strike Source, Counter Strike 1.6 and Counter Strike Condition Zero. On this topic, in Alan Wake for the Xbox 360, the flashbang is an offensive weapon which inflicts heavy damage on enemies.