Lightning is an item from the Mario Kart series. Usage results in the player's adversaries shrinking. A similar item has the same effect, called the Thunderbolt. Players being affected by a Star will be unaffected by the Lightning.
Mario Kart series
Super Mario Kart, Mario Kart 64 and Mario Kart: Super Circuit
In Super Mario Kart, Mario Kart 64 and Mario Kart: Super Circuit, upon Lightning's use, all other racers in a race are instantly turned tiny. A tiny racer is much slower than their regular counterpart, and is squished upon contact with a bigger racer. The racers turn to regular size after so many seconds, starting with the racer in eighth place. Being squished by another racer has the same effect as being squished by a Thwomp. Also, in Mario Kart: Super Circuit, using the item will stop the music from playing, but once the shrunken players return to normal size, the music restarts from the beginning.
Mario Kart: Double Dash!! and Mario Kart DS
In Mario Kart: Double Dash!! and Mario Kart DS, the Lightning acts very similarly to how it works in Mario Kart 64. The only difference is that players who are tiny are not squished by their larger adversaries, but rather knocked aside fiercely. If you hit an item at the right time, you won't get shrunk. In Mario Kart: Double Dash!!, if an item thrower is dragging on the ground and you or an opponent uses Lightning, then they will shrink and say their dragging sound effects, which are faster and more high-pitched.
Mario Kart Wii
In Mario Kart Wii, the Lightning affects all other racers, similarly to how it has in other Mario Kart games. The item, however, can be cancelled out by use of the new Mega Mushroom item, which when used while tiny, brings the racer who used it back to normal size. If the racer already is giant upon the Lightning's use, they will become regular sized. The POW Block (another new item) acts similar to the Lightning Bolt, except it shakes up the other racers and doesn't shrink them. It can also be avoided if the player is in the air or if they press the tricks button.
Mario Kart 7 and Mario Kart 8
In Mario Kart 7 and Mario Kart 8, the Lightning acts similar to how it works in Mario Kart DS and Mario Kart Wii (The farther the player who got hit by the Lightning, the quicker he/she will return to normal size). However, the Lightning still affects racers when they get shot out of a cannon or while gliding.
Super Smash Bros. series
Super Smash Bros. Brawl
In Super Smash Bros. Brawl, Lighting makes an appearance as an item. When touched, it causes several different possible outcomes to occur. This item may make the character who touched it shrink, or make the opposing characters shrink. It may also make them giant, or even just the character who touched it giant.
Super Smash Bros. for 3DS/WII U
In these games, they work absolutely the same as in Brawl. They float in the sky and slowly fall to ground or platform. If the effect is shrink, the ones who have Super Mushroom in use will only get them back to normal, and the ones that are trapped with Poison Mushroom will not be affected of it. If the effect is supersize, the ones who have Super Mushroom in use, will not be affected and the ones that are trapped with Poison Mushroom will grow back to normal size they always have normally in battles.