expecto patronum

http://en.wikipedia.org/wiki/Expecto_patronum#Expecto_Patronum_.28Patronus_Charm.29

Expecto Patronum (Patronus Charm)

Pronunciation: ex-PEK-toh pa-TROH-nuhm

Description: Conjures an incarnation of the caster's innermost positive feelings, such as joy, hope, or the desire to survive, known as a Patronus. A Patronus is conjured as a protector, and is a weapon rather than a predator of souls: Patronuses shield their conjurors from Dementors, and can even drive them away. A Patronus "cannot feel despair, as real humans can, so Dementors can't hurt it."^[16] The conjured Patronus protects the witch or wizard that summoned it, obeys his or her commands, and fades away shortly after it is no longer required. When conjured, a Patronus appears silvery, ethereal, and semi-transparent. Improperly formed Patronuses range from momentary formless bursts of silvery mist, to poorly-defined forms which are easily defeated or quickly dissipate on their own. A full-fledged (or corporeal) Patronus takes on a fixed animal form that is often significant to the witch or wizard casting the charm. Patronuses summoned by a particular person have been known to change, such as Tonks' patronus. Rowling has said that Snape was the only Death Eater to be able to produce a Patronus. According to her this is 'because a Patronus is used against things that the Death Eaters generally generate, or fight alongside. They would not need Patronuses'.^[17] According to Fantastic Beasts and Where to Find Them, the Charm is also the only known defensive spell against Lethifolds.

Seen/Mentioned: First seen in Prisoner of Azkaban when a Dementor appears in the Hogwarts Express, and Hermione says that Remus Lupin repelled the Dementor by casting a silvery object from his wand. Harry's corporeal Patronus first appears in a Quidditch game, and it is used by other characters throughout the rest of the series.

Notes: Dumbledore has devised a method of using Patronuses to deliver messages putting it into the exclusive use of the Order of the Phoenix. Members of the Order are the only wizards who know how to use their spirit guardians to send messages to one another. According to Rowling, the Patronus is "an immensely efficient messenger" as it is not hindered by physical obstructions or dark matters. Each Patronus has a special quality and appearance that is different and easy to recognise, which makes it clear which Order member has sent the message. In addition, since no one can conjure another person's Patronus, this method of communication does not carry the risk of passing fake messages.^[18] It is noteworthy that in Deathly Hallows, McGonagall creates three Patronuses simultaneously to call for Professors Flitwick, Sprout, and Slughorn.

Suggested Etymology: Expecto Patronum is Latin for "I await a protector".^[19] It is related to "pater" (father) and Harry's Patronus indeed takes the same form as that of his father's animagus form (a stag).