Buttpad
Also: Recoil Pad
The rubber or adjustable pad at the rear of the stock that cushions recoil and sets the rifle's length of pull.
The buttpad is the cushioned surface at the very back of the stock, where the rifle meets the shoulder pocket. A soft rubber or gel pad spreads the impulse of recoil over a wider area and a longer instant, which reduces felt kick and helps the shooter stay relaxed through a string of fire. The grippy face also keeps the rifle from sliding out of position as the shoulder absorbs each shot.
Beyond comfort, the buttpad is where length of pull is set, since spacers behind it or a sliding mount move the contact surface fore and aft to fit the shooter. On precision stocks the pad often adjusts for height as well, working with the comb so that a square mount and a consistent cheek weld come together on every repetition.