Backstop
Material or terrain behind the target that will reliably stop a bullet.
The backstop is what catches the bullet after it leaves the target. At an outdoor range, it’s the earthen berm. At an indoor range, it’s the bullet trap. When shooting outside a range, the shooter is responsible for confirming a competent backstop — soil, dense rock, or a hillside — before firing. Rule 4 (“be sure of your target and what is beyond it”) is about the backstop.