Nonverbal spell

"Your adversary has no warning about what kind of magic you are about to perform, which gives you a split-second advantage."

- Hermione Granger quoting Standard Book of Spells, Grade 6

A nonverbal spell is a spell which is performed without saying the incantation out loud. Performing spells nonverbally is very difficult and requires a good deal of practice, as it requires concentration and mental discipline alone. Hogwarts School of Witchcraft and Wizardry begins teaching nonverbal magic to students in their sixth year in Defence Against the Dark Arts, Charms, and Transfiguration. Harry Potter noted that several students in his class looked "like they had received an overdose in U-No-Poo", a substance that causes constipation, while attempting nonverbal spells.

Effectiveness
Some spells, such as Levicorpus, are easier to perform nonverbally than others. Most spells, however, seem to be less effective than normal when the incantation is not said. For example, the unnamed curse that critically injured Hermione Granger during the Battle of the Department of Mysteries might have been fatal had Antonin Dolohov not been silenced when he used it, according to Hogwarts nurse Poppy Pomfrey.

Known practitioners
The following is a list of wizards and witches known to have nonverbally performed spells that are usually performed with an incantation.
 * Albus Dumbledore
 * Antonin Dolohov
 * Arthur Weasley
 * Barty Crouch Jr.
 * Bellatrix Lestrange
 * Bill Weasley
 * Draco Malfoy
 * Filius Flitwick
 * Fleur Delacour
 * Fred Weasley
 * Hermione Granger
 * Female Hufflepuff
 * Kingsley Shacklebolt
 * Lord Voldemort
 * Luna Lovegood
 * Madam Malkin
 * Madam Pomfrey
 * Marietta Edgecombe
 * Minerva McGonagall
 * Nymphadora Tonks
 * Peter Pettigrew
 * Quirinus Quirrell
 * Remus Lupin
 * Rubeus Hagrid
 * Severus Snape

Possible uses

 * Fred and George Weasley cast Expelliarmus nonverbally when they were surreptitiously disarming Zacharias Smith at the first meeting of Dumbledore's Army.
 * Harry Potter performed nonverbal magic when he cast his Patronus during the Battle of Hogwarts.
 * Molly Weasley killed Bellatrix Lestrange with a nonverbal magic during the Battle of Hogwarts.

Behind the scenes

 * In many of the Harry Potter films, especially in the later ones, combat and duels are portrayed as being conducted almost entirely nonverbally, such as the Duel in the Ministry Atrium and Harry's final duel with Voldemort during the Battle of Hogwarts. This is probably done to keep the duels fast-paced and dramatic.
 * In the Harry Potter video games, non-verbal magic is commonly used, as pronouncing every spell could have been annoying.
 * In the film adaption of Harry Potter and the Philosopher's Stone, Quirrell used a Nonverbal wandless spell to start up some fire in order to stop Harry Potter from running.
 * Hermione Granger may have used Wingardium Leviosa in the film version of Harry Potter and the Order of the Phoenix to levitate the prophecies to smash one of the Death Eaters chasing them.

Appearances

 * Harry Potter and the Philosopher's Stone 
 * Harry Potter and the Philosopher's Stone (film)
 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Chamber of Secrets (film)
 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 2 
 * LEGO Harry Potter: Years 1-4
 * Harry Potter and the Deathly Hallows: Motorbike Escape

Notes and references
Sortilège informulé