The rearward force or momentum of a firearm against the shooter’s shoulder or hand when fired.
When a gun is fired, the rapid forward acceleration of the bullet causes an opposite rearward reaction of the firearm, pushing the firearm back toward the user. This is a classic example of Newton’s third law of motion. Depending on the power of the cartridge and the mass of the firearm, the recoil can range from barely noticeable to considerably uncomfortable. The recoil reaction is mostly rearward, but also includes other movement of the firearm, including muzzle climb.