A superjump is a type of glitch where, under certain conditions, a character will be sent far upwards in their neutral falling animation. The force is often enough to launch the character past the top blast line (since not in hitstun, they will not be KO'd). If a solid platform gets in the way while the character is going upwards, they will float under it for several seconds (not jumping and not being in hitstun means no bouncing off ceilings). Notable superjumps include the Boomerang superjump, Flight of Ganon, super death jump and the size change super jump. Shield jumping is also a type of superjump, however, it is not a glitch and does KO Jigglypuff.
Superjump physics
Most superjumps follow the same principle: If an action is interrupted between a large upwards force and a comparably large downwards force, the downwards force will not happen and the uninhibited upwards force will fling the character skywards. For example, if Kirby is using his up throw and the character he's grabbing vanishes after the upwards force but before the downwards force, Kirby will be sent very high.
A similar mechanic enabled double jump canceling in SSB and SSBM.