A midair jump, commonly referred to as a "double jump", is a jump that can be used once every time the user is airborne. Just like the single jump, double jumping is achieved by hitting up on the d-pad/control stick or hitting the jump button. Every character can use a mid-air jump in the Smash Bros. series, regardless if such an ability exists in their original series. Most characters can use a single mid-air jump, while some characters can use up to three or five. A character's mid-air jump(s) are restored upon landing, grabbing a ledge, or being KO'd and respawning (once the character "lands" on the Revival platform).
Double jumps are a lot more varied in animation than single jumps. Some characters twist, others flip, and there are some double jumps that do something completely different. Like jumps from the ground, characters have a different animation if they are jumping backwards. For example, Mario spins around when performing a regular midair jump, but does rapid backward somersaults when performing a backwards midair jump. If a character flips, their collision bubble moves accordingly, though compared to the act of jumping in the first place this does little in-and-of-itself to help evade attacks, and has no effect on the jump itself.
Certain characters will create a brief "ripple" in the air at the start of their mid-air jump (pictured above), though this is purely visual.
Certain characters can inflate themselves (Kirby, Jigglypuff, King Dedede) or flap their wings (Meta Knight, Charizard, Pit) to jump multiple times in the air. Characters with multi-jumps can turn around when in midair by using them. These multi-jumps become useless (or nearly useless for some) if the character is metal. Characters with multi-Jumps have good air speed (with the exception of Meta Knight and King Dedede) and are usually light-weight (with the exception of Charizard and King Dedede).