Riddikulus

"'R-r-riddikulus!' squeaked Neville. There was a noise like a whip-crack."

- Neville casts this spell.

Riddikulus is a charm that is used in defence against a Boggart. It causes the creature to assume a form that is humorous to the caster, thereby counteracting the Boggart's ability to terrorise. Boggarts are defeated by laughter, so forcing them to assume an amusing form is the first step to defeating them. However, because Boggarts are ammortal, this spell does not truly dispel them, similar to the Patronus Charm.

Casting
To cast, the spellcaster must first acknowledge what they fear the most (as it will most likely be what the Boggart will take the form of) and then visualize it into something amusing. The problem is, because the Boggart would have assumed said form at the moment of direct encounter, the one attempting to use this charm would have lost concentration after seeing the Boggart manifest their worst fears, making this spell useless. As such, it is always advisable to first take one step at a time when practicing this enchantment.

History
Professor Remus Lupin taught this to his third year Defence Against the Dark Arts class. Thanks to Lupin's teaching, Harry Potter was able to use this against a Boggart in the Triwizard Maze in 1995.

Etymology
"Riddikulus" is an adaptation of "ridiculous" as well as of ridiculum (Latin, "joke") and ridere (Latin, "to laugh").

Behind the scenes

 * The effect of Riddikulus varies throughout the series. In Harry Potter and the Prisoner of Azkaban, it has its stated effect of forcing a Boggart into an amusing form and otherwise does not harm it. In Harry Potter and the Goblet of Fire, however, Harry uses the spell to make a Boggart-Dementor explode into a wisp of smoke. Various anecdotes on Pottermore describe Riddikulus transforming Boggarts in a variety of ways, not all of which being directly related to making the Boggart funny.
 * In LEGO Harry Potter: Years 1-4, only Ron and Hermione learn this spell due to Harry being locked out of the class while Lupin is teaching it to his class. Harry instead learns Expecto Patronum from Lupin to defeat his Dementor boggart, and in return Ron and Hermione do not learn Expecto Patronum.

Appearances

 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Prisoner of Azkaban (video game)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Order of the Phoenix
 * LEGO Harry Potter: Building the Magical World
 * LEGO Harry Potter: Years 1-4