Grip Safety
A spring-loaded safety on the rear of the grip that must be depressed (by a firm grip) before the firearm will fire.
A grip safety is a paddle on the back-strap of a handgun (the 1911 is the canonical example; the Springfield XD and HK P7 are modern adopters) that the shooter must depress with a firm firing grip. It’s a passive safety — engaged automatically when held normally — and prevents discharge if the trigger is somehow pressed by something other than a deliberate firing grip.