Riddikulus: the Boggart-Banishing Charm

Riddikulus is the spell used to banish Boggarts: it forces the creature into a more comical form, provoking the laughter that leads to its eventual banishment. 1

A Boggart mirrors the fear of whoever faces it, so the joke has to fit the fear. The spell's creator must make the creature into a figure of fun; if the caster can laugh aloud, the Boggart disappears at once. 2 3

In Harry Potter and the Prisoner of Azkaban, Remus Lupin teaches the charm to his Defence Against the Dark Arts class. Neville faces the Boggart first and turns his fear of Snape into his grandmother's clothes and a hat topped with a stuffed vulture. 1 4