Flashbang grenade

A Flashbang Grenade (also called a stun grenade) is a special type of grenade used by the UNSC. Unlike Fragmentation Grenades, these grenades are not intended for lethal use. Flashbangs use chemical reactions to produce an extremely bright flash of light and a very loud concussion blast when deployed, stunning targets, but doing very little physical damage to them. The primary purpose of a stun grenade is to temporarily blind and deafen the enemy. If the victim directly observes the flash, a still frame of what is observed by the victim will be seen for up to ten seconds, at which point their eyes will have returned to their unstimulated state.

Due to widespread usage within 21st Century armed forces, it has likely been used since before the UNSC's conception, though the earliest recorded use of it was 2531, as seen by its use in Halo Wars. The Flashbang and Stun Grenades' use in combat scenarios has not be overlooked, serving to confuse, disorient, or momentarily distract a potential threat for five to ten seconds.

Triva

 * Halo 3 possesses a piece of UNSC equipment serving a similar function as a Flashbang, the Flare, which is used to blind all nearby players (even if they can't see it), though it lacks the deafening effect of a flashbang. It does, however, have a noise that can drown out certain other sounds.
 * In Halo Wars, the Flamethrower unit has an upgrade that has a stun grenade ability.