The Imperturbable Charm (incantation unknown) is a charm that creates an invisible magical barrier on an object, such as a door. This barrier bounces objects off of it, and muffles sounds. An object enchanted in this manner is said to have been "Imperturbed".
Molly Weasley used this charm on the kitchen door at 12 Grimmauld Place to prevent the children from eavesdropping on Order of the Phoenix meetings using Extendable Ears. Ginny Weasley was taught by Nymphadora Tonks how to test if something had been Imperturbed by flicking Dungbombs at the suspected target and seeing if they bounced off.
Behind the scenes
In the film adaptation of Harry Potter and the Order of the Phoenix, this charm is not used on the kitchen door, and because of this Fred, George, Harry, Hermione, Ron, and Ginny are able to eavesdrop on the Order of the Phoenix meeting that took place on Harry's first night at 12 Grimmauld Place.