In the Super Smash Bros. series
The Lightning Bolt appears as an item in Brawl and Smash 4. When used, it will shrink every other character to minimal size, similar to its function in the Mario Kart series. However, the item can backfire as well, resulting in the player who picked up the item grow small instead or making the other characters' size increase (similar to the effects of the Super Mushroom, except that it's twice as effective). Unlike most other items, the Lightning Bolt requires a character to simply make contact with it in order to gain its effect. Any character affected by knockback while being struck by the Lightning Bolt is still affected by knockback after the shrink/growth occurs. Similar phenomena occur with the Super Mushroom and Poison Mushroom.
Super Smash Bros Brawl
An item that goes off the moment you touch it. It may shrink every other character on the screen. Tiny players suffer a reduction in their attack power and become easier to send flying. After a set amount of time, all the characters will return to their normal sizes. In the Mario Kart series, all players but the item user shrunk, and got flattened when run over.
Super Smash Bros. 4
The Lightning Bolt's primary effect comes from the Mario Kart series. The Lightning Bolt first appeared in Super Mario Kart. Racers at the back of a race have a chance of acquiring a Lightning Bolt (sometimes named Thunderbolt), which upon use instantly strikes all other racers, spins them out, and causes them to shrink. Shrunk racers have lowered top speed and may be easily knocked around or even flattened when run over. Beginning in Mario Kart DS, the length of the shrink time depends on what place each opponent was in when the item was used, with those at the back regrowing near-instantly and those are the front staying shrunk for much longer. In Super Mario Kart, Mario Kart 64, Mario Kart 7 and Mario Kart 8, tiny players can get flattened by normal-sized players. The item cannot backfire as it does in Brawl.
In Brawl, if a character uses the Lightning Bolt on Zelda or Sheik and either character Transforms, this glitch makes the size of the Lightning Bolt user permanent until either Zelda or Sheik gets KO'd or picks up another size-changing item.