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Thunder is Pikachu's down special move in the Super Smash Bros. series and Pichu's in Super Smash Bros. Melee, based on the Electric-type move in the Pokémon series. Pikachu or Pichu calls down a bolt of lightning that strikes any opponents in its path to damage them. If the bolt strikes Pikachu or Pichu, any nearby opponents are damaged by this as well. In Super Smash Bros. the lightning falls from the top of the screen, but in Super Smash Bros. Melee a cloud appears above the player at a fixed height relative to their position. In addition, in Super Smash Bros. Brawl onward, the move sends Pikachu into the air if it strikes Pikachu while in midair, and in Super Smash Bros. for Nintendo 3DS / Wii U the move can meteor smash at a certain point in addition to the shockwave being able to now damage opponents.
Pichu's version of Thunder is able to hit multiple times, making the complete strike stronger than Pikachu's, though the bolt hitting Pichu deals less damage and instead damages Pichu itself.
In Super Smash Bros. for Nintendo 3DS / Wii U this move has been given two customizable variants. The first one is Thunder Burst, which instead of bringing down a bolt of lightning causes Pikachu to discharge electricity from itself to damage opponents for multiple hits. The second one is Distant Thunder, which causes the cloud to appear at a higher distance, increasing the duration of the move and dealing more damage closer to the cloud.