In an Instagib match all players start with a Super Shock Rifle, a weapon that kills any enemy player with just one shot. All other weapons and ammo are removed from the game, as are pickups (sans specific items such as the Jump Boots), although players may still be equipped with a Translocator. The Super Shock Rifle cannot be dropped.
In 2003/2004, there's a special variant of the Instagib mutator, called Zoom Instagib, whose alternate fire is a zoom function like that of the Lightning Gun. It's inspired by a third-party Unreal Tournament mutator of the same name.
The popularity of the Instagib mode spread to plenty of other FPSes (mostly free, open-source) including Nexuiz, Xonotic, OpenArena, and last but definitely not least, and very especially, Quake Champions.