Fidelius Charm

The Fidelius Charm is a complex and powerful spell that allows someone or something to be hidden from the sight of anyone besides specific individuals for an indefinite period of time. Those specific people are the Secret-Keeper and the individuals the Secret-Keeper reveals the information to. However, if a person, rather than an inanimate object, is being hidden, they cannot be secret-keepers (otherwise, the Potters would have designated themselves as such in 1980).

The secret being hidden is stored within the very soul of the Secret-Keeper. Only they have the power to reveal the secret; not even those who have been told the secret can reveal what they know. The information may be passed on through words or writing. If someone has not been given the information, they will be unable to locate the hidden secret, even if they staring right at it. When the Secret-Keeper dies, everyone who has been told the secret becomes a Secret-Keeper. If the Secret-Keeper dies without having revealed the secret to anyone, then the secret dies with them, forever. The information will be hidden for all time (unless, of course, there were multiple secret-keepers before the death).

The Fidelius Charm was used to protect James, Lily and Harry Potter from Lord Voldemort at their home in Godric's Hollow. They designated their Secret-Keeper as Sirius Black, but then switched to Peter Pettigrew as they thought it would be less obvious. Pettigrew betrayed them to Lord Voldemort, who found and murdered the Potters. Only Harry survived the attack.

In 1997 when Albus Dumbledore was killed, the members of the Order of the Phoenix automatically became Secret-Keepers for their headquarters at 12 Grimmauld Place. Unfortunately, this included Severus Snape, whose allegiance was doubtful at the time. As such, the Order was forced to abandon their headquarters.

Soon afterwards, the Death Eaters evidently became aware of 12 Grimmauld Place's approximate location, as they took to watching the gap between houses 11 and 13 when on the lookout for Harry Potter. The house remained invisible to them until Yaxley grabbed onto Hermione Granger's arm while she, Harry and Ron Weasley Disapparated away from the Ministry of Magic to 12 Grimmauld Place, where they had been hiding. Because Hermione had been rendered a Secret-Keeper by Dumbledore's death, she inadvertently took Yaxley past the protections of the Fidelius Charm. The trio subsequently abandoned the house as a hideout. Arthur and Bill Weasley used this charm on their households in 1997 when they went into hiding.

Behind the scenes

 * Harry Potter and the Deathly Hallows contained a few discrepancies regarding the Fidelius Charm. When the Weasley family was forced into hiding, Bill told them that Arthur was Secret-Keeper, and also told them that they were staying at Aunt Muriel's. Since Arthur was Secret-Keeper, Bill shouldn't have been able to tell the trio about the Weasley's whereabouts. Also, when Harry, Ron, and Hermione were captured and taken to Malfoy Manor, Ron told Dobby where to take the hostages when the house-elf came to rescue them. Although it is possible that Bill and Arthur were both Secret-Keepers for Muriel's, and that Ron had been made into an additional Secret-Keeper for Shell Cottage, this was never specified.

Etymology
"Fidelis" is Latin, meaning trusty and faithful.

Fidelitas