Muggles' Guide to Harry Potter/Magic/Glisseo

Overview
The Glisseo charm flattens the treads of a staircase, effectively turning it into a slide.

Extended Description
We first see this charm in action in Harry Potter and the Order of the Phoenix when Ron, upset at what he believes to be a betrayal of the newly created Dumbledore's Army, attempts to fetch Hermione from her dormitory. After he has taken about three steps on the staircase, it turns into a slide under him, depositing him on the floor of the Common Room. Hermione later says that evidently the Founders felt girls were more trustworthy than boys, which is why the boys' dormitories are not similarly rigged to prevent girls from entering.

We do not hear the incantation, though, until Harry Potter and the Deathly Hallows. There, in the Battle of Hogwarts, the Trio have taken refuge behind the tapestry at the top of a flight of stairs. A pair of Death Eaters spot them, and Hermione uses Glisseo to turn the stairs into a slide to hasten their getaway.

Analysis
Additionally, "glisseo" in Latin translates directly to "slide".