Patronus Charm

"A Patronus is a kind of positive force, and for the wizard who can conjure one, it works something like a shield, with the Dementor feeding on it, rather than him. In order for it to work, you need to think of a memory. Not just any memory, a very happy memory, a very powerful memory… Allow it to fill you up... lose yourself in it... then speak the incantation "Expecto Patronum"."

- Remus Lupin teaching Harry Potter the Patronus Charm

A Patronus Charm is an insubstantial animal protector created by the advanced Patronus Charm spell, and one way to defend against Dementors and certain other Dark creatures. The spell requires the use of a wand, concentration on a powerfully happy memory, and the incantation "Expecto Patronum". Presumably, certain physical gestures with the wand are also required, but they have not been specified. The Patronus Charm is advanced magic and difficult to master, especially in emotionally difficult circumstances.

The Patronus spellbook, owned by Remus Lupin, includes information and instruction on the Patronus Charm.

Form
A Patronus that is completely and fully formed, or corporeal, takes the shape of fairly solid-looking animals; a non-corporeal Patronus appears only as wisps of silvery mist. It is a very complex charm and many qualified wizards and witches have trouble with it. Harry Potter is one of the youngest known wizards to cast a Patronus; he was taught how to do so in 1994 at the age of thirteen by Remus Lupin. In a Dumbledore's Army lesson, Harry taught the members how to use the charm. Some were even successful in casting a corporeal form, though Harry said this might be because there was no Dementor to make them frightened. However, some members were later shown to be able to use corporeal Patronus later on in the presence of Dementors.

The Patronus Charm, like Animagus forms, has been said to reflect the personality or feelings of the witch or wizard. They are, however, subject to change if the caster goes through an emotional upheaval of some sort. For example, Nymphadora Tonks's Patronus changed from its previous form to that of a four-legged creature which others guessed was a werewolf because then it would reflect her love of Remus Lupin, himself a werewolf. Another example is Severus Snape, whose Patronus was that of a doe, the same as Lily Evans, though this may have been the case all along rather than a change from its original form, as Snape was in love with Lily for most of his life. If a wizard is an Animagus and can summon a corporeal Patronus, the two may take the same form, as was the case for Minerva McGonagall and James Potter. Most Patronuses take the form of an ordinary animal, but magical creatures, such as Albus Dumbledore's phoenix Patronus, are not uncommon.

The Charm
"Make it a powerful memory, the happiest you can remember. Allow it to fill you up… Just remember, your Patronus can only protect you as long as you stay focused… Think of the happiest thing you can."

- Harry Potter teaching the Patronus Charm to Dumbledore's Army

In order to cast this charm, one must think of an extremely happy memory or idea, and then say the incantation: "Expecto Patronum". Because of this, negative emotions can compromise a wizard or witch's ability to cast a Patronus. For instance, Harry Potter, Ron Weasley, and Hermione Granger all had difficulty casting their Patronuses when under the negative influence of Salazar Slytherin's Locket, one of Lord Voldemort's Horcruxes, and after witnessing the death of Fred Weasley in battle.

However self-confidence may also play an important role in casting the spell. In 1994 Harry is able to cast the spell successfully, driving off a large number of Dementors (which according to Severus Snape is only achievable by a very powerful wizard) thus saving Sirius and a younger version of himself, seemingly without recalling a happy memory when he realises that he has already done it.

Uses
There are two known uses for the Patronus charm. The first and main use of the Patronus is to drive off certain Dark creatures such as Dementors and Lethifolds, and is the only known charm that will work against them. As Dementors feed on happy memories to render humans to drown in sorrow and sadness, the Patronus acts as a shield of sort, of which the Dementor would try to feed off of instead - the Patronus is comprised exclusively of positive feelings and cannot comprehend negative feelings, so the Dementor's influence cannot harm it. A non-corporeal Patronus only impedes Dementors, while a full-fledged one can drive them away.

The other use was devised by none other than Albus Dumbledore, who invented a way of using the Patronus to deliver messages. This source of communication is believed to be exclusive to the members of the Order of the Phoenix. Using the Patronus for communication offers great advantages in security, as Patronuses identify the caster and are Dark-proof.

Severus Snape used his Patronus to lead Harry Potter to a pool in the Forest of Dean in 1997, where he had laid Godric Gryffindor's Sword for Harry to retrieve.

Minerva McGonagall used her Patronus to call for help to the other heads of houses in 1997 when she found out Harry Potter had returned to Hogwarts in search of Rowena Ravenclaw's Diadem.

Etymology
Patronus means "protector" or "guardian" in Latin, specifically reflecting the entire role that the Patronus Charm plays. In archaic Latin, it meant "father", which is very interesting, considering that Harry Potter's Patronus is the same as his father's Patronus and Animagus form, a stag. The Latin word exspecto or expecto means "I watch for" or "I await", thus the charm's incantation roughly translates into "I await a protector." The Charm's plural is 'Patronuses'

Behind the scenes

 * Both the Patronuses of James Potter and Minerva McGonagall match their respective Animagi form. These are the only characters whose Patronus and Animagus forms are known.  It is uncertain whether all Patronuses will match the form of the Animagus, but so far, an instance of differing forms has not been revealed.
 * In the first chapter of Harry Potter and the Deathly Hallows, a white peacock appears in the gardens of Malfoy Manor. Some fans believed this bird to be Lucius Malfoy's Patronus. However, J.K. Rowling stated that no Death Eater except Severus Snape could conjure a Patronus. This means that the peacock was in fact a living creature. It is stated in Harry Potter and the Deathly Hallows that the creature was in fact an albino peacock.
 * In the video game adaptation of Harry Potter and the Prisoner of Azkaban, the more moderate Expecto Patronum is a ball of light (before cast, a white ring goes up Harry's arm and if it reaches top of the wand, Harry has to try the spell again) and is used only to attack Dementors. At the climax of the game, a stag (Harry's Patronus) jumps out of the wand and the player must control it to drive away all the Dementors nearby attacking Harry and Sirius.
 * It is possible that Patronuses can foreshadow a marriage. Lily Evans' Patronus was a doe, the female counterpart to her husband James Potter, whose was a stag. Also, Hermione Granger's Patronus is an otter. Her husband, Ronald Weasley lived with his family in Ottery St. Catchpole, though this last one might just be a coincidence as J.K. Rowling's favorite animal is an otter.
 * The lecture that Harry gives on Patronuses to Dumbledore's Army in the film version of Harry Potter and the Order of the Phoenix is extremely similar to the lecture Remus Lupin gave him in the film version of Harry Potter and the Prisoner of Azkaban.
 * In Harry Potter and the Deathly Hallows: Part 1, Dolores Umbridge's Patronus seems to emanate a shield that keeps the Dementors separated from everyone in the courtroom. In addition, Kingsley Shacklebolt's Patronus is described in the novel as taking the form of a lynx, however in the film it is depicted as a shimmering comet of light from which springs silvery images of witches and wizards screaming in terror as Kingsley's voice details the death of Scrimgeour and the fall of the Ministry.
 * In Lego Harry Potter: Years 1-4, Voldemort, Lucius Malfoy, Walden Macnair, Barty Crouch Jr., and an unnamed Death Eater can all cast the spell, despite the fact that Rowling stated that no Death Eater besides Severus Snape could produce a Patronus.
 * According to W.O.M.B.A.T., it is possible that Patronuses vary in strength according to which animal's form they take.

The Patronus in the films

 * In the film adaptation of Harry Potter and the Prisoner of Azkaban, Harry's and Remus' non-corporeal Patronuses are more like a shield. In addition, his corporeal Patronus, a stag, sent out a sort of omni-directional pulse, which drove the Dementors away, rather than running them down (likely because of the number of Dementors and their flight abilities in the film).
 * In the film adaptation of Harry Potter and the Order of the Phoenix, his Patronus physically attacks the Dementors by charging at them, as depicted in the books. Also, Patronuses produced by the members of Dumbledore's Army make noises and are capable of interacting with physical objects, such as Ron's dog Patronus knocking over Neville Longbottom.
 * There was a noted change in appearance of the Patronus in the Order of the Phoenix movie. In movies 5-7, the Patronus is very whispy and airy, while in the third film it looks more substantial and solid.

Appearances
thumb|300px|right|Patronus being cast.
 * 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
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Order of the Phoenix (video game)
 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Deathly Hallows
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 2
 * Harry Potter and the Deathly Hallows (video game)
 * Lego Harry Potter Years 1-4
 * Harry Potter: Spells

Notes and references
Sortilège du Patronus Patronus Патронус Suojelius