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 form 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.

Form
A Patronus that is 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 1993 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 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 unknown.

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.

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. 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.

Etymology
Patronus means "protector" or "guardian" in Latin, reflecting the role the Patronus Charm plays. In archaic Latin, it meant "father", which is interesting, considering that Harry Potter's Patronus is the same as his father's Patronus and Animagus form. 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 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 version 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.

Behind the scenes
"He had a very strange dream. He was walking through a forest, his Firebolt over his shoulder, following something silvery white. It was winding it's way through the trees ahead, and he could only catch glimpses of it between the leaves. Anxious to catch up with it, he sped up, but as he moved faster, so did his quarry. Harry broke into a run and ahead, he heard hooves gathering speed. Now he turned a corner into a clearing and -" - Harry Potter and the Prisoner of Azkaban
 * 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, Ron Weasley lived with his family in Ottery St. Catchpole.
 * 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 Prisoner of Azkaban, Harry had something of a precognitive dream about Snape's silver doe. It could be said that some of the stories' plot can be revealed through Harry's dreams.

Appearances
thumb|300px|right|Patronus being casted.
 * 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

Notes and references
Sortilège du Patronus Patronus Патронус Suojelius