"Dumbledore ...ran onto the field as you fell, waved his wand, and you sort of slowed down before you hit the ground."
-- Hermione Granger (PA9)
Arresto Momentum is an incantation that slows the speed or stops a falling object or person to keep them from harm.
References from the canon
Old French arester "to stay, stop" + Latin momentum = "movement"
This is probably the non-verbal spell used by Dumbledore to slow Harry's fall when he was attacked by Dementors while playing Quidditch in Third Year (PA9).
In the film for Deathly Hallows Part 2: Hermione uses the incantation "Arresto Momentum" to save them when they fell from the Gringotts cart. However, the book does not use this incantation, instead it only mentions that the spell was the Cushioning Charm. So is the Cushioning Charm's incantation "Arresto Momentum"?
The films are not considered to be canon, since they were not written by Rowling. Her mention of the term "Cushioning Charm" in Quidditch Through the Ages and the accompanying illustration show that the spell creates what is essentially an invisible pillow on the handle of a broom, which is very different from the effect we see with "Arresto Momentum": slowing something's fall. That would suggest that the scriptwriter's use of the term "Cushioning Charm" in the film is an error.
Adding to the confusion, the Cushioning Charm is shown as slowing someone's fall and is given the incantation "Molliare" in Cursed Child -- but that play, while considered to be canon even by Rowling herself, wasn't written by her.
So which is it? Is the Cushioning Charm the same as Arresto Momentum? Are there two spells both called that name which different effects, or possibly variations of the basic spell? There is no way to be sure.
From the Web
Harry Potter Wiki: Slowing Charm