Fidelius Charm

From Hpwiki

Taken from the Latin word "fidelis", which means trusty and faithful, the Fidelius Charm is an extremely complicated spell that is used to conceal information through the means of one person - a Secret-Keeper. Such a charm has been used on the likes of Lily and James Potter - to conceal their whereabouts when Voldemort was after them - and also to conceal the whereabouts of the Order of the Phoenix location - Number Twelve, Grimmauld Place.

"An 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." - Harry Potter and the Prisoner of Azkaban, Chapter 10: The Marauder's Map.