Splash Damage (occasionally referred to as Radius Damage) is a term that refers to refer to damage taken by players or objects in the area surrounding a point of weapon impact. Splash damage is most often dealt by explosive weapons such as Grenade Launchers and Rocket Launchers, and to a lesser extent the Plasma Gun from Quake 3 Arena. Splash Damage can also be caused by an explosion from a Radioactive Container. BFG10K in Quake 3 Arena is also unique for dealing splash damage, while BFG10K in Quake 2 and Dark Matter Gun in quake 4 cannot.
Splash damage is a skill that can be used to damage multiple people with one shot, or to propel them through the air. It's a common strategy to fire at an enemy's feet with the Rocket Launcher, as a direct hit is not necessary with the large radius of splash damage. Damage dealt will lessen the further away the target is from the point of impact.
In Quake 4, the setting for splash damage is slightly altered in id Tech 4 engine, direct hits will not deal extra splash damage, this applies to Grenade Launcher and Rocket Launcher.