A double jump (also called an air jump or a midair jump) is a jump that can be used once every time the user is airborne. Just like the grounded jump, double jumping is achieved by hitting up on the Control Stick or hitting the jump button. Every character has at least one double jump in the Smash Bros. series, regardless of whether such an ability exists in their original series, while some characters can use up to five.
A character's double jump(s) are restored upon landing, grabbing a ledge, being KO'd and respawning, or being grabbed by an opposing character (even if the character being grabbed does not touch the ground). Unlike most recovery moves, double jumps are not given back to the player upon being put into hitstun. This can be catastrophic to a character trying to return to the stage if they have already used their double jump(s) and an opponent edge guards their attempt to recover.
In contrast with jumps from the ground, each character only has one height of double jump, whereas the player can control grounded jumps to be either short hops or full hops. The exception to this is with characters with more than one midair jump, as these usually decay in height with each successive jump.
Double jumps are a lot more varied in animation than single jumps. Some characters twist, others flip, and some characters inflate themselves (Kirby, Jigglypuff, King Dedede) or flap their wings (Meta Knight, Charizard, Pit, Dark Pit, Bayonetta, Banjo & Kazooie). During these animations, all characters cast a double circle platform beneath them that disappears after the jump is completed (except for R.O.B. in Brawl, in which a smoke ring is formed instead from the base of his Robo Burner; this does not waste any fuel from his recovery). Like jumps from the ground, characters have a different animation if they are jumping backward. For example, Mario spins around when performing a regular midair jump, but does rapid backward somersaults when performing a backward midair jump. If a character flips, their collision bubble moves accordingly, though this has no effect on the jump itself. Some characters will create a brief "ripple" effect in the air at the start of their mid-air jump.
Delayed double jumps
Most characters have their vertical speed instantly set to their given value upon using their double jump, providing an immediate height boost. Certain characters instead have double jumps that have a short pause before applying a slower and constant upwards boost, controlled by animation rather than an immediate velocity change. Depending on the game, the floatiness of such second jumps allows for several techniques such as double jump canceling and rising aerials. Ness, Mewtwo and Lucas have this type of double jump. Yoshi had a delayed double jump in Smash 64, Melee and Brawl, while Peach had it in Melee before losing it in Brawl.
Multiple double jumps
Most characters can only jump once while in the air; however, some characters can jump multiple times without touching the ground. Most characters cannot turn around in mid-air using their double jump, but all characters with multi-jumps can turn around when in midair by using jump while holding away from the direction the character is facing on the Control Stick (Yoshi is also able to do this, despite only having a single midair jump).
However, these specific fighters tend to gain substantially less height per jump than the jumps of fighters with only one midair jump. There is usually also a brief delay between the end of one jump and being able to start the next. Usually, this delay is small enough for the character to gain a decent amount of height with all of their jumps, but if the character is metal, these multi-jumps become useless or nearly useless.
Characters with multi-jumps usually have poor air speed (with the exception of Jigglypuff, Meta Knight in Smash 4, and Ridley).
In Melee, Captain Falcon and Ganondorf gain another midair jump after using Falcon Kick and Wizard's Foot, respectively. In the initial release of Super Smash Bros. for Nintendo 3DS, a bug allowed Robin and Kirby, upon copying Robin, to regain all of their respective midair jumps after charging Thoron; this bug was removed in version 1.0.4.
In games from Melee onward, characters with multiple midair jumps cannot drop through soft platforms during any of their midair jumps until their animation completes, even if holding down on the Control Stick or after fast falling. This does not apply to their ground jumps, and is unlike other characters, who can all drop through platforms during any portion of their midair jump.
Multiple double jumpers
Double jumping is a common mechanic in many platforming video games. Occasionally it is given as a power-up or an upgrade, while other times it is just a natural ability. Fighters who can double jump in their native games include Banjo and Kazooie, Bayonetta, Daisy, Kirby, Samus, Sonic, Wario, and Yoshi.