Thunder (かみなり Kaminari) is Pikachu and Pichu's down special move. When used, the Pokémon yells as a bolt of lightning drops from the sky, striking the user if its horizontal momentum has not carried it out of the way (and if there is no platform in the way). Upon hitting the user, a large shockwave is created with stronger knockback than the lightning itself. It will also grant momentary invincibility if it hits 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 user can act more quickly if they are not stuck.
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. Though the move is powerful, it has considerable ending lag.
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. Pikachu's Thunder's bolt strike has greater horizontal knockback than its clone Pichu's, and is otherwise the same as it was in Super Smash Bros., except it no longer has 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 it. In Smash 64, the bolt does 12% damage, with Pikachu's body spark doing 16% damage. In latter games, the bolt does 10% while the spark deals 17%. For Pichu, the bolt deals 16% damage if all hits connect (total of 4), and the spark causes 14% damage.
In Super Smash Bros. Brawl and Melee, 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. Also, in Brawl, Thunder can pass through platforms that are close enough to the cloud. This is best seen in Temple if Pikachu uses Thunder on the bottom portion of the stage, where in Melee the attack would fail.
Also in Brawl and Melee, the offscreen cloud portion of the thunder attack can be used as a KO hit if the player being hit is high enough in the air.
In Brawl, grounded Thunder KOs at 75%.
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.
The move is confirmed to appear in the upcoming Super Smash Bros. 4.
 Reverse Thunder
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.
Thunder is based off a move of the same name in the Pokémon RPGs. Thunder is well-known as a very powerful and mainstream special Electric-type attack with base power of 120 as well as being the strongest Electric-type attack in the first generation of the Pokémon games (and one of the strongest in the generations afterwards), and for its paralysis rate of 30%. It is also known for its relatively low accuracy of 70% (outside of weather effects), which hinders its efficiency. However, it can even hit Pokémon that are preparing to use Fly, while most other attacks fail against them. In the Smash Bros. games, it is hard for Thunder to hit enemies at the sides of the user, or with the spark the Pokémon produces, but unlike other moves, it can perfectly hit enemies that are at a great distance above the user (having an almost infinite vertical range in Smash 64).
In every generation, Pikachu is able to learn Thunder by leveling up, being of the latest moves it learns. Pichu needs a TM25 (Technical Machine) to learn it, however.
 Use by other Pokémon
Call thunderbolts down from the sky. They won't pass through terrain obstacles.
 Description from the Brawl Instruction Booklet
Call forth a huge bolt of lightning that shoots straight down. It does not travel through platforms.
 See also