Flashbang grenade
From Halopedia, the Halo wiki
The Flash-Bang Grenade or Stun grenade is a special type of grenade used by the UNSC. Unlike Fragmentation Grenades, these grenades are not necessarily lethal. They produce an extremely bright flash of light and a very loud noise when they go off, but do very little physical damage to a target.
There are a variety of uses for these. They have been used in military exercises by the UNSC, at least since 2525. They are also occasionally used in combat as well, serving to confuse, disorient, or momentarily distract a potential threat for five to ten seconds. Although a flash-bang can seriously degrade the combat effectiveness of affected personnel for up to a minute, they are still vital in any hit-and-run operation.
In Halo 3 a similar piece of equipment, the Flare, is used and has the same look as the Power drainer and Radar jammer. It will blind looking players temporarily.
