Hydro Pump is a Greninja's up special move in its playable Super Smash Bros. appearances. The move originates from the Pokémon franchise as a powerful Water-type move.
Upon using the move, Greninja launches itself up into the air with two blasts of water from its palms. The player can launch Greninja in any direction with each blast by angling the control stick or Circle Pad in the appropriate direction. The move can also damage opponents and push them away.
In Super Smash Bros. for Nintendo 3DS and Super Smash Bros. for Wii U, the first custom variant for this move is High-Capacity Pump, which enables Greninja to fire water pumps that are easier to control, and offer greater distance overall than Hydro Pump, but the move travels slower than normal, and is not able to deal any damage. The second custom variant is Single-Shot Pump, where Greninja launches itself straight up with a single water pump, which covers more vertical distance than Hydro Pump. The move's angle can't be changed, however.
Hydro Pump is also used by other Pokémon in the Super Smash Bros. series. In Super Smash Bros. and Super Smash Bros. Melee, Hydro Pump is the move used by Blastoise when it is summoned from a Poké Ball. In Super Smash Bros. Brawl and Super Smash Bros. Ultimate, Squirtle uses Hydro Pump in the Pokémon Trainer's Final Smash, Triple Finish.