When used, the Pokémon yells as a bolt of lightning drops from the sky, damaging opponents in the Thunder's path. Upon hitting the user, a large shockwave is created, which deals more damage and knockback than the regular bolt, along with granting temporary invincibility to the user (this is best seen when using it on a Blast Box, where the Blast Box is destroyed, but the user isn't harmed). The lightning blast stalls the user in place for a while, so they can act more quickly if they are not stuck.
Thunder is based on the move of the same name in the Pokémon games, introduced in the first generation. Thunder is a powerful special Electric-type attack, being the strongest Electric-type attack in the first generation. It has a notable drawback, however: its relatively low accuracy (but certain weather effects can decrease or increase its accuracy). However, it can hit Pokémon that are preparing to use Fly, while most other attacks fail against them, which is reflected in the fact that in the Smash Bros. games, Thunder can hit airborne opponents.
In every generation, Pikachu is able to learn Thunder though leveling up, usually being one of the last moves it learns that way. Pikachu can also learn the move via TM25 in every generation so far; this TM is also the only way Pichu can learn the move.
In Super Smash Bros., this move has seemingly infinite vertical range and thus can be often used to score KO's when an opponent is near the upper blast line. This technique is called "Thunderspiking", and can lead to much lower-damage KO's. As a result, Pikachu players try to integrate it into their combos as often as they can, typically after an up smash. Even though the move is powerful, it has considerable ending lag. Regardless, it is considered one of Pikachu's best moves. Does 12% damage, 16% for Pikachu's body spark.
In Super Smash Bros. Melee, the move strikes from a cloud at a fixed height relative to the user (this is often not visible unless the stage is rather high). For example, if Pikachu or Pichu is under the upper-middle platform at Hyrule Temple and uses Thunder, one can see the cloud from where the thunder is generated, but the lightning will not go through the platform, making the attack useless in that particular location. The thunder portion of the attack is considered a projectile and can be reflected back at Pikachu or absorbed for health. The explosion cannot, however. Pikachu's Thunder's bolt strike has greater horizontal knockback than Pichu's, and is otherwise the same as it was in Super Smash Bros., minus the infinite height. Pichu's bolt strikes multiple times and has slightly more upward knockback than Pikachu's, however. Pichu also incurs 3% damage if the bolt strikes it, making it the only electrically charged attack Pichu has that might not damage itself. Pikachu's bolt does 10% while the spark deals 17%. For Pichu, the bolt deals 3, 3, 3, and 5% damage, 16% damage if all hits connect. The spark causes 14% damage.
An interesting property of this is that if this attack connects with Pikachu in mid-air, Pikachu will bounce upwards a bit. Due to its startup time, it won't aid much in recovery, but is useful to guard break, especially against spikers. If used near the edge of a stage, the Thunder can create a wall of electricity to prevent a recovering opponent (especially if Thunder Jolts are sent out beforehand), best if they try to go for the ledge.
In Brawl, Thunder can be reversed by pushing the control stick/d-pad in the opposite direction in the first few frames. Though useless on the ground, when performed in midair with momentum, the momentum will be reversed, pushing Pikachu in the opposite direction.
The move returns in Super Smash Bros. 4. It now has a meteor smash hitbox, however, this is more harmful than good. The meteor smash is near unreliable as a KO move, and it makes Thunderspiking much harder to perform. In addition, it's very difficult to actually use the meteor smash to lead into follow-ups. However, its new hitbox allows opponents to be hit by its shockwave hitbox around Pikachu. It also passes through platforms. With good practice, it has become more useful over time, despite its nerfs.
Thunder also gets two variations: "Thunder Burst," where electricity discharges from Pikachu to launch opponents, and "Distant Thunder," which starts higher than normal, with a sweetspot where it begins and a sourspot where it ends.
This remains unchanged from Brawl. However, it is less useful and it still involves meteor smashing.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Thunder returns as Pikachu and Pichu's down special move in Super Smash Bros. Ultimate. While Pikachu's version remains similar to how it worked in SSB4, Pichu's variant, while retaining its recoil damage, functions differently from Melee, as the thunderbolt now only hits once, and has gained a meteor smash hitbox, akin to Pikachu's Thunder.
Use by other Pokémon
There are also Pokémon other than Pikachu and Pichu that have used Thunder: