Lightning (also known as Thunderbolt and Thunder) is an Item. Racers will receive these when they're either in, or close to, last place.
When the racer uses Lightning, all other players in front of them will be electrocuted by a storm cloud that appears over them, making it a great recovery Item. When they are struck, they spin out and lose any of there Items (depending on the game). They also shrink down to a very small size, which makes them decrease in speed (another good reason for you to want this Item!). The effect wears off depending on what game and place you are in. In some Mario Kart titles, if a player has been electrocuted by Lightning and a regular sized character runs you over, the racer will get squished, losing even more speed. That would just be bad luck.
In Mario Kart 64, if the player runs over a player who is small while he or she is big, they will be squished for a while even if the launcher is a lightweight and the lightning-stricken player is a heavyweight. If two small people run into another they both will be squished. If they hit an obstacle such as a Thwomp they will be pushed back and be squished.
In Mario Kart: Super Circuit, when this item is used, the music stops playing and then restarts after all shrunken drivers return to normal size. The shrunken drivers will spin out if coming into contact with another driver.
In Mario Kart: Double Dash!!, it won't affect the player if he or she is currently indisposed (such as having just been hit by another item or fallen off a cliff).
In Mario Kart DS and Mario Kart Wii, the effect of being hit by a Thunderbolt wears off quicker the lower in the rank you are. For example, someone hit by one while in 7th/11th place will return to normal almost immediately, while those in 1st or 2nd place will remain shrunk for much longer. Also, the background music of the track will be sped up and raised in pitch. In Mario Kart Wii, a racer who was super-sized from using a Mega Mushroom would be returned to normal size when struck by the Thunderbolt instead of shrunken completely.
In Mario Kart 7, the Thunderbolt acts similarly as in Mario Kart 64, in the fact that racers who are small can get run over.
In other games
The Thunderbolt was an item featured in Super Smash Bros. Brawl. If a fighter touched it, either they, their opponents, or everyone would shrink, or they, their opponents, or everyone would grow.