Skull Bash (ロケットずつき, Rocket Headbutt) is a damage-dealing Normal-type move introduced in the first generation.
It's popular among trainers as it has both a high power and accuracy rating, though the Pokémon is only allowed to perform this attack 15-24 times until they run out and must refill. This was also a move in the Super Smash Bros. series, in which Pikachu and its pre-evolved form Pichu could perform it in Super Smash Bros. Melee (Pikachu, Pichu) and Super Smash Bros. Brawl (Pikachu).
In the Super Smash Bros. series, the player controlling either Pikachu or Pichu would be able to execute this attack once they pressed B while pressing left or right on the joystick. Once you released, the Pokémon would shoot to the right or left, depending on which direction you desired. The longer you held down the B button, the longer they would go. Luigi of the Mario series has a similar move in the Super Smash Bros. series called the Green Missile, though it was slightly different in that it would often times misfire and shoot the character off of the stage.