A flash grenade was a type of non-lethal weapon used by Starfleet in the early 24th century. A flash grenade produced a brief, intense flash of light capable of temporarily disorienting most humanoid species.
History and specifics
Starfleet flash grenades were first used against Cardassian targets running a raid on an illegal Cardassian base on the moon of Delavi III in 2328. In that encounter, the Cardassians proved quick to recover from the weapon's effects. (CoE eBook: Turn the Page)
These were occasionally also called photonic grenades because of their output of light, though there was no technical connection to the photonic torpedo. (SCE eBook: Fatal Error)
Mirror universe
In 2409, Val Shon was armed with flash grenades while attempting to rescue Tress Shra and others imprisoned in a Tholian labor camp on Dewa III. When Val detonated one, his prime universe counterpart Captain Va'Kel Shon wished he'd been warned about their intensity, which were stronger than in his reality and temporarily disoriented him as well as several Tholian guards. (STO - Star Trek Magazine short story: "Mirror Image, Part 2")