Fidelius Charm

The Fidelius Charm is a very powerful spell used to hide things or places, or to conceal secrets. The spell is described as “(a)n immensely complex spell involving the magical concealment of a secret inside a single, living soul. The information is hidden inside the chosen person, or Secret-Keeper, and is henceforth impossible to find — unless, of course, the Secret-Keeper chooses to divulge it” (PA10). The workings of this powerful spell are something of a mystery.

References from the canon

• Used to try to protect Lily and James Potter from Voldemort. "As long as the Secret-Keeper refused to speak, You-Know-Who could search the village where Lily and James were staying for years and never find them, not even if he had his nose pressed against their sitting room window!" (PA10) Unfortunately, Peter Pettigrew was chosen as Secret-Keeper, and he betrayed the secret.
• Dumbledore used the Fidelius Charm to hide number twelve Grimmauld Place, the headquarters of the Order of the Phoenix. He himself was the Secret-Keeper for the Order (OP6). Note that he once mentioned it in front of the Dursleys (HBP3).
• Snape could refer (at least indirectly) to the fact that a Fidelius Charm applied to the headquarters of the Order of the Phoenix, but since he was not the Secret Keeper he could not mention the name of the place (HBP2).
• "When a Secret-Keeper dies, their secret dies with them, or to put it another way, the status of their secret will remain as it was at the moment of their death. Everybody in whom they confided will continue to know the hidden information, but nobody else...
• "In other words, a secret (e.g., the location of a family in hiding, like the Potters) is enchanted so that it is protected by a single Keeper (in our example, Peter Pettigrew, a.k.a. Wormtail). Thenceforth nobody else - not even the subjects of the secret themselves - can divulge the secret. Even if one of the Potters had been captured, force-fed Veritaserum or placed under the Imperius Curse, they would not have been able to give away the whereabouts of the other two. The only people who ever knew their precise location were those whom Wormtail had told directly, but none of them would have been able to pass on the information (JKR)."
• After the death of a Secret Keeper, each of the people to whom he or she had confided the secret will become a Secret Keeper. The power of the Fidelius Charm will be diluted more and more as more and more people are Secret Keepers (DH6).
• Mention of a Secret Keeper (DH9).
• At the Burrow, Arthur Weasley was the Secret Keeper, while Bill was the Secret Keeper for Shell Cottage (DH24).