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 paraphrasing from the 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 "liked 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.
 * Fleur Delacour
 * Antonin Dolohov
 * Albus Dumbledore
 * Marietta Edgecombe
 * Filius Flitwick
 * Hermione Granger
 * Bellatrix Lestrange
 * Remus Lupin
 * Draco Malfoy
 * Minerva McGonagall
 * Peter Pettigrew
 * Quirinus Quirrell
 * Severus Snape
 * Lord Voldemort
 * Fred Weasley
 * Arthur Weasley
 * Kingsley Shacklebolt

Possible Uses

 * Fred and George Weasley may have performed nonverbal magic while practicing Expelliarmus by surreptitiously disarming Zacharias Smith at the first meeting of Dumbledore's Army.
 * Harry Potter may have performed nonverbal magic when he cast his Patronus during the Battle of Hogwarts.
 * Molly Weasley may have killed Bellatrix Lestrange with a nonverbal spell during the Battle of Hogwarts; an incantation was not mentioned, but the spell itself was not mentioned either, nor were any of the other spells they used during the duel.