Jostling is a form of collision detection present throughout the Super Smash Bros. series. Typically, it is understood as the effect wherein two objects are overlapping, and as a result each object is slowly pushed away from each other. When multiple characters attempt to occupy the same space, they push away from each other. This effect also applies to other elements such as items, enemies, and summoned Pokémon and Assist Trophies, though some of these only jostle each other and not things in other groups. The effect tends to be stronger on the ground, particularly in Ultimate between two opposing fighters.
In Super Smash Bros., only characters can jostle; items cannot. This partially results in the Green Shell glitch, which is caused when multiple Green Shells are positioned exactly on top of each other. All characters will push each-other with equal strength. Melee would introduce the ability for items to partake in the effect. In Super Smash Bros. Brawl, while the effect itself is unchanged, enemies and bosses in the Subspace Emissary tend to use non-damaging and pushing hitboxes to amplify the effect. This behaviour also notoriously features in Little Mac's forward smash and KO Uppercut in Smash 4, in addition to a couple of his other moves.
In Super Smash Bros. Ultimate, jostling on the ground has been heavily reworked. Characters on the ground who are not teammates can no longer walk or run past each other at all, outside certain actions such as dodges, and because of this the mechanic can now be utilised offensively in normal gameplay. When one character runs into another, they will push said character forward at a fraction of their running speed, and will not be able to move through them. How strongly one character can push another character is determined by their weight; the higher a fighter's weight is, the more strongly they can push other fighters, and the more strongly they resist being pushed themselves.
Additionally, it is now possible for the strength of a fighter's jostle to temporarily be modified while using certain moves. Currently, the only character to utilize this is Isabelle with Fishing Rod. Furthermore, some moves temporarily modify the area on the attacker capable of jostling, seemingly to prevent blindspots without the use of windboxes.