Places
Room of Requirement
The Room of Requirement is a Hogwarts room that appears, on the seventh floor opposite a tapestry, when someone needs it and walks past three times thinking of that need. Dumbledore’s Army trains there. Draco Malfoy uses another form of it to hide a Vanishing Cabinet. In the last book it becomes a hiding place for students, and then the fire that destroys the diadem Horcrux.
In the books
Harry Potter and the Order of the PhoenixChapter: Dumbledore's Army
Harry needs a place for the Defense group to meet, and the room provides one. Dobby is the one who tells him how the room works.