Wikia

Ascendio
Incantation	Ah-SEN-dee-oh
Type	Charm
Hand Movement	Point wand in the air above you
Effect	Lifts caster high into the air

Ascendio is a charm used to lift the caster high into the air, or if the user is submerged then it will propel them to the surface.

Possible uses

On 4 June, 1992, Quirinus Quirrell may have used a non-verbal, wandless version of this spell when he flew across the Mirror of Erised Chamber to Harry Potter, and then held him down to the floor. He also may have learned Voldemort's ability to fly.

Harry Potter only uses this spell in the film adaptation of Harry Potter and the Goblet of Fire, in the book he manages to swim to the surface without using a spell, only just barely.
Ascendio may be closely related to the spell Alarte Ascendare which was used in the film adaptation of Harry Potter and the Chamber of Secrets . Both spells lift their targets high up into the air, with main difference being the target - Ascendio launches the caster into the air while Alarte Ascendare launches a target directed at by the wand.

"Ascendio" comes from the Latin word "ascendo", meaning "to climb".