Lightning (also known as Thunderbolt and Thunder) is an item. Racers will receive these when they're either in, or close to, last place. Usage will make all other competitors smaller, slower, unable to use certain items and have high-pitched in voices.
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.