A pocket door is a sliding door that disappears, when fully open, into a compartment in the adjacent wall. Pocket doors are used for architectural effect, or when there is no room for the swing of a hinged door. They usually travel on rollers suspended from an overhead track, although some also feature tracks or guides along the floor. Both single- and double-door versions are used, depending on how wide an entry is desired.

(http://en.wikipedia.org/wiki/Pocket_door)