Being stunned or dazed (referred to as FuraFura in Super Smash Bros. Melee's debug menu and in-game files for Brawl, Smash 4 and Ultimate, after the Japanese onomatopoeia for dizziness), an effect known as dizziness in Brawl, is a special condition that leaves a character immobile for a short period of time.
A stunned character is dazed for a few seconds and can't perform any actions until the condition ends. Button mashing reduces the duration of the effect, and suffering any amount of knockback ends it prematurely. In Smash 64 and Melee, characters can change to other actions immediately after their stun time ends, which can cause unsuspecting opponents attempting to punish them to get punished themselves instead. From Brawl onward, however, every character has an animation for regaining consciousness after their stun ends (known internally as FuraFuraEnd), which lasts 50 frames in Brawl, and 33 frames from Smash 4 onward, giving players a better visual cue and effectively adding more vulnerability time for the stunned character.
Stun is accompanied by a cartoony tweet sound effect that loops until the condition is stopped, as well as small stars that occasionally pop up around the character's head on top. This sound effect debuted in Melee, and its volume was increased in Brawl. In all games, characters also have a flashing red overlay applied to their model; in Smash 4 and Ultimate, however, this only happens when a character's shield is broken. Also, in Smash 4, a halo of stars appears around a stunned character's head. In Ultimate, the number of stars is reduced to 2, but a solid orange halo appears around the character's head as well.
It is impossible to be stunned while airborne. Possibly as a result, any airborne state induced upon a stunned character will cancel the stunned state immediately and they will enter a normal midair state.
The stun effect is weight independent, but the amount of knockback dealt can still be affected by knockback modifiers and knockback resistance (such as from the Metal Box). A stun effect hitbox will fail to stun the target if it doesn't deal any knockback, or doesn't deal more knockback than the target's armor threshold (such as Bowser's Tough Guy mechanic in Smash 4 and Ultimate). However, stun duration itself is independent of knockback.
The formula for stun time varies across games, as well as depending on how the stun is inflicted. From Brawl onward, this does not consider the animation for regaining consciousness, effectively adding more vulnerability on top of the actual stun time.
While a character is stunned, including the animation for regaining consciousness, hitting them with a stunning move does not extend the duration, instead simply causing flinchless damage. From Brawl onward, this applies for 70 additional frames after breaking free from stun. Characters also cannot be stunned in midair; attempting to do so causes the stunning move to inflict standard knockback instead, which in Melee is also the case for using Disable on an already stunned opponent.
For a shield break, the formula in Smash 64 and Melee is
Button mashing reduces the duration of shield break stun by 4 frames per input in Smash 64, and 3 frames per input in later games (5.4 for buttons in Smash 4 and Ultimate, but they cannot be inputted as quickly as stick inputs); compared to other status conditions, this leaves players highly vulnerable during a shield break, even with exceptionally fast mashing.
Attacks with the stun effect
In Melee, the only attack that can stun opponents is Mewtwo's Disable, which uses the formula
In Brawl, attacks that stun opponents use a general formula of
As a result, in addition to characters' rank in a battle being a more important factor for stun time, attacks have increased stun time based on their knockback amount rather than the opponent's percent (though the attack's base damage is a factor in knockback calculation, and thus still relevant to stun time). However, due to Mewtwo getting cut in Brawl, there are no longer any playable characters that can use a stunning move without items (Wario-Man's uncharged Wario Waft can stun, but requires the Smash Ball).
In Smash 4 and Ultimate, the general stun formula is slightly modified to
Mewtwo returns as a playable character after Brawl, alongside its Disable move, making stun once again part of a playable character's moveset without items. However, Disable uses another formula for its stun duration, separate from the general one. This formula is
In Ultimate, the placing factors are also different; h (stock deficit) is equal to 0.2 for a -1 difference, and 0.45 for -2 or lower, while r (stock lead) is equal to 1.2 for a +1 difference, and 1.4 for +2 or higher. As a result, Disable stuns opponents for 4 more frames with a -1 difference in stocks (or points), 8 more frames with a -2 difference or lower, 19 less frames with a +1 difference, and 22 less frames with a +2 difference or higher. Without the placing factors, the formula can be simplified to
Causes of stunning
By enemies and bosses
In Wrecking Crew, getting trapped in a barrel and taking heavy damage will occasionally result in the trapped character being stunned upon breaking out. This can only be done with low-knockback attacks, however.