Priority is a property of hitboxes in the Super Smash Bros. series that describes their behavior upon interacting with other hitboxes. While the priority of the hitboxes produced by normal ground attacks follows a set of rules that is dependent upon the amount of damage they deal (the law of high and low priority), different rules apply to those of normal aerial attacks, and certain special moves when performed in the air. Furthermore, some hitboxes and attacks, such as shots from Fox's Blaster, possess a special kind of priority called transcendent priority, which ignores the rules of normal priority. Rebound is a term used to describe an animation that occurs when two direct grounded moves with normal priority mechanics are within 9% of each other when they collide. The length of this animation changes based on the damage output of both attacks.
Priority in the Super Smash Bros. series is not to be confused with the general term "priority" that is used to gauge the relative usefulness of a move with regards to its speed, the size or duration of its hitbox, or any other property it might have (e.g. armor or intangibility/invincibility around part of or the entire body). The concept of priority also governs how individual hitboxes and objects react, not attacks as a whole. For example, an attack can be cancelled if its only non-transcendent hitbox is outprioritized, regardless of the status of its other transcendent hitboxes. In addition, attacks being cancelled via priority rules is on a per-object basis, meaning that an attack that is outprioritized can still damage other players or objects if it hits at the same time.
Normal priority describes the set of rules that apply to normal attack hitboxes, that is, any attack hitbox that is not classified as transcendent. As such, the majority of standard attacks, aerials, special attacks, and projectiles contain hitboxes with normal priority.
Ground attacks and projectiles
The hitboxes of normal ground attacks follow the law of high and low priority. This means that they interact with each other in terms of the damage they deal, which in turn classifies each as having "high" or "low" priority. When two ground attack hitboxes overlap, they will collide, and will either cancel each other out, or one will override (out-prioritize) the other. This collision is signified by a white "bubble", and in Melee, a distinct "ting" sound, as if swords are clanging, is also heard. If two actual swords clash, such as Link and Marth's blades, a more realistic "ting" sound is heard. The law of high and low priority functions in terms of a damage (priority) range of 9%. On these terms, one ground attack must deal 9% or more additional damage than another attack if it is to out-prioritize it, hence, the "priority range" in Smash Bros games is 9% (note that in the original Super Smash Bros., the priority range is instead 10%).
If one attack hitbox is above the priority range of another, that is, deals 9% or more than the other, the stronger hitbox out-prioritizes the weaker one, and the weaker attack is cancelled by the stronger one. However, if two colliding ground attack hitboxes are within the priority range, meaning that the difference in damage dealt by the two hitboxes is less than or equal to 9%, they will "clash", and both will cancel out, delivering no damage to either character. Thus, normal attacks with "high" priority are those whose hitboxes deal a large amount of damage, and are therefore more inclined to out-prioritize other attacks, while those with "low" priority deal little damage.
The same rules apply when ground attack hitboxes overlap normal projectile hitboxes.
While most normal projectiles follow the rules of ground priority, Snake's Remote Missile, Diddy Kong's Peanut Popgun, and King Dedede's Waddle Dee Toss behave differently. While the 8% range does apply to Dedede's Gordo, its hitbox behaves like an airborne attack as it cannot be cancelled out. For example, if Captain Falcon's forward smash collides with a Gordo (19% vs. 23%), the clash bubble will appear, and the forward smash will be cancelled out while the Gordo is not. However, since both hitboxes are within the priority range, the Gordo will go right through Captain Falcon without harming him, and continue along its natural path. It will, however, still possess its damaging hitboxes, and can still harm any other character it happens to touch along its route.
Snake's Remote Missile has a damagable hitbox and behaves like a thrown item, such as a capsule. Hence, when opposing hitboxes collide with the Remote Missile, they will damage it instead of colliding with it. If the Remote Missile takes enough damage, it will be destroyed. The peanuts from Diddy Kong's Peanut Popgun behave the same way, but any colliding hitbox, even if it only deals 1%, will destroy the peanuts.
Different rules apply to the hitboxes of normal aerial attacks. When a normal aerial attack hitbox overlaps that of a normal ground attack or another normal aerial, the attacks cannot collide or clash and the law of high and low priority does not apply. If an overlap between the hitboxes of two such attacks occurs, both will persist, irrespective of each other, and will damage their foes if they contact them.
However, the hitboxes of aerial attacks can collide with normal projectiles, in which case the law of high and low priority functions. However, aerial attack animations cannot be cancelled out, and will continue even if out-prioritised.
Rebound refers to the usual outcome when two different hitboxes connect without one having priority over the other. The outcome is the animations of both attacks are interrupted and both fighters are briefly caught in a unique "rebound" animation where neither can perform any action until the animation is finished. The length of the animation increases proportionately to the damage the hitbox would have dealt. This means the fighter that performed the weaker attack would get out of rebound before the other and have frame advantage. The calculation for rebound duration depends on the game, but "R" always means rebound frames and "d" always means damage dealt. In Melee, the calculation is R=(6.6+(d)*0.558). In Smash 4, the calculation is R= Floor((d*15/8+)7.5).
Exceptions to normal priority
Considered both airborne and grounded
*The second hitbox of Zero Suit Samus' up smash behaves like an aerial. It may be out-prioritised by another character's aerial, but the up smash's animation and subsequent hitboxes cannot be cancelled.
The hitboxes of the above attacks can collide with the hitboxes of other attacks regardless of if they're ground or aerial attacks. When such a collision occurs, the clash bubble will appear and the outcome will follow the law of high and low priority. However, airborne attacks cannot be cancelled out, including airborne special moves. Thus, while the results of a collision will follow the law of high and low priority, the appearance of such a collision will differ to that of two colliding ground attacks.
If one of the above ground attacks collides with an aerial, or one of the above aerials collides with a ground attack, and the ground attack is below the priority range of the aerial, the ground attack will be cancelled and the airborne attack can damage the wielder of the ground attack. If the two hitboxes are within the priority range, the ground attack will be cancelled out, and, since it is an aerial attack, the animation of the airborne attack will persist. However, the airborne hitbox becomes harmless, and the wielder of the ground attack will not at all be knocked or damaged by the airborne hitbox, regardless of whether it connects or not.
If the hitbox of a ground attack is above the priority range of a colliding airborne hitbox, both attack animations will persist, but the airborne hitbox will become harmless, while the ground attack is still able to damage. Thus, if the hitbox of the ground attack connects with the body of the character who used the airborne special, this character will take the damage and knockback of the ground attack. A notable example of a special move that can collide with the hitboxes of other attacks while airborne is Meta Knight's Mach Tornado. If performed on the ground (grounded), it can be cancelled out, but if airborne, it cannot be cancelled. Mach Tornado is a multi-hit move that deals 1% per hit during the main portion of the attack.
Moves that cannot rebound
Some attacks cannot rebound after colliding with an attack within the priority range (also known as trampling); in other words, they will continue their animations as if they did not collide at all, but both hitboxes still cancel each other out if they are within the 9% range. This can be advantageous for moves with this property that are either multihitting in nature, very fast, or very damaging (which would increase the length and therefore vulnerability of the rebounding animation), but is disadvantageous if the move's endlag is longer than the length of the opponent's rebounding animation. Additionally, two moves that cannot rebound that are used against each other are more prone to damaging both characters instead of cancelling out their hitboxes.
Strangely in Brawl, the hitbox flag which should determine whether a move can rebound or not is discarded as soon as the hitbox is generated. This means that moves in Brawl which are not supposed to rebound (such as Jigglypuff's dash attack) will rebound anyway. This was most likely a coding error.
* Should not rebound but rebounds anyway, due to the anti-rebound flag not working properly in Brawl.
** Does not rebound in later games, but the effect is nullified by the move gaining transcendent priority.
Non-character objects with complex animations
A select few attacks in SSB4 will collide with other attacks, but can only nullify projectiles within the 9% range and will always lose a trade with regular hitboxes regardless of the 9% rule, much like a grab. For example, Bayonetta's up smash cannot be used as effectively as other up smashes to hit opponents in the air, as any hitbox that comes into contact with it will render the up smash inert, and if the hitbox is close enough to Bayonetta at the moment of colliding, only Bayonetta will take damage. However, if the opposing hitbox is a projectile or another attack with this property (i.e. Lucario's Aura Sphere or another Bayonetta's up smash), both characters will take damage as if one of them used an aerial hitbox (but the collision bubble will still appear). The overarching pattern among these attacks is that they are character-specific props that feature complex animations beyond simply moving or rotating, and thus it is possible (but not proven) that the game short-sightedly does not attempt to make them capable of trading with attacks from other animating elements (i.e. characters).
Because they cannot override regular hitboxes and it is not always clear to players whether a hitbox is attached to a character or a prop, these attacks are often perceived as having "low priority", and some moveset repositories such as Kurogane Hammer list them under such term.
Transcendent priority (also known as transcending priority) refers to hitboxes that cannot clang with other hitboxes, meaning they won't cancel out, or be cancelled out by, other hitboxes, even other transcendent hitboxes. Transcendent hitboxes will always win exchanges, but are more likely to trade hits and are unable to cancel out projectiles. In Smash 64, transcendent priority only existed for certain projectiles (such as Fox's Blaster) but from Melee onward, it could exist for any kind of attack.
Attacks with transcendent priority