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 jostle strength; the higher a fighter's jostle strength is, the more strongly they can push other fighters, and the more strongly they resist being pushed themselves. This value loosely correlates with a fighter's weight, however there are outliers. The speed of a fighter's walk or dash determines the rate at which they can apply the force of their push in a direction, faster movement applying force at a faster rate. When fighters collide while moving in opposite directions a combination of the pushing forces' strengths and rates of applying force determines their travel speed and direction. With great enough speed a fighter can apply their jostle force so rapidly that it can match or overtake that of a heavier fighter pushing back more strongly at a slower rate, but a slower and heavier fighter can do similarly to a lighter and faster one should they be moving fast enough (i.e. a dashing Sonic can push back an opposing dashing Shulk despite being lighter, but the Shulk can push back the Sonic when using the Speed Art despite still being slower). Should the opposing forces be equivalent, the fighters will run in place (i.e. A dashing Pit and an opposing dashing Dark Pit will do this because of their equivalent jostle strengths and dash speeds, as will a dashing Sonic and an opposing dashing King K. Rool due to the combined forces of their differing weights and dash speeds evening out).
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.
The following is a table listing every fighter's jostle strength, along with their weight for reference. Note that the two are loosely correlated, but not directly correlated.