Muggles' Guide to Harry Potter/Magic/Imperturbable Charm

Overview
An Imperturbable Charm, cast on a door or window, prevents approach to the door by anything.

Extended Description
We first hear of this charm when Fred and George are talking about trying to use their Extendable Ears to listen in on meetings of the adult members of the Order. Ginny tells them not to bother, there is an Imperturbable Charm on the door, and that she found this out by tossing Dungbombs at it: none of them had reached the door.

When the family goes to visit Arthur in the hospital for the first time after his injury, the adults there present, Arthur, Molly Weasley, Mad-Eye Moody, and Tonks, send the youngsters out of the ward so they can have a quick discussion. Fred and George again produce Extendable Ears, saying that they hope nobody has thought to put an Imperturbable Charm on the door. It turns out nobody has.

In Harry Potter and the Half-Blood Prince, when Harry, Ron, and Hermione have followed Draco Malfoy to Borgin and Burkes, Ron produces Extendable Ears to allow them to listen in on what Draco is saying, and says that he hopes the store is not protected by an Imperturbable Charm.