A pocket door is a sliding door

Sliding door

A sliding door is a type of door which opens horizontally by sliding, whereby the door is either mounted on or suspended from a track. Types of sliding doors include pocket doors, Arcadia doors, and bypass doors...

 that disappears, when fully open, into a compartment in the adjacent wall. Pocket doors are used for architectural

Architecture

Architecture is both the process and product of planning, designing and construction. Architectural works, in the material form of buildings, are often perceived as cultural and political symbols and as works of art...

 effect, or when there is no room for the swing of a hinge

Hinge

A hinge is a type of bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation. Hinges may be made of flexible material or of moving components...

d 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.

Furthermore, installing a pocket door rather than a hinged door can add an average of ten square feet of floor space, according to building expert Tim Carter, who considers the pocket door "one of the top ten most overlooked items when many architects and builders plan a home". The doors were particularly common in Victorian homes to close off such areas as sitting rooms or dens; however, as architectural tastes changed, many of the hardware manufacturers went out of business. With improvement in the hardware and the growth of the market for condominiums and town homes, there has been a resurgence of interest in this space-saving feature. Modern residential uses include bathrooms, closets, laundry or utility rooms, or home offices. A wall-hung variation called an "open pocket door" may be used where in-wall installation is impractical; this version is recommended for homes with disabled residents due to greater ease of opening compared to traditional hinged, pull-open doors. One downside to pocket doors is hidden parts and hardware, which can make them difficult to replace or repair when something goes wrong. Fixing the problem might require removing the door and trim and opening up the wall.

The 1953-'54 Darrin sports car used pocket doors.

Additional reading

"Pocket Door Installation," DIY to the Rescue http://www.diynetwork.com/diy/hi_bedrooms/article/0,2037,DIY_13904_2684878,00.html

"Installing Pocket Doors"
http://www.pocketdoorshop.co.uk/infopage.aspx?aid=18