Longman Dictionary of Contemporary English
Wiktionary
n. A door that opens and closes by sliding to the side; sometimes one of a pair.
WordNet
n. a door that opens by sliding instead of swinging
Wikipedia
A sliding door is a type of door which opens horizontally by sliding, usually parallel to a wall. Sliding doors can be mounted either on top of a track below or be suspended from a track above and some types 'disappear' in a wall when slid open. There are several types of sliding doors such as pocket doors, Arcadia doors, and bypass doors. Sliding doors are commonly used as shower doors, glass doors, screen doors, and wardrobe doors.
A sliding door is a type of door is mounted on or suspended from a track for the door to slide, usually horizontally. It is a feature predominately relegated to minibuses and buses to provide a large entrance or exit for passengers without obstructing the adjacent pathway between the vehicle and any adjoining object or the side(s) of passenger and commercial vans so as to allow a larger unobstructed access to the interior for loading and unloading. Sliding doors are often used in mini MPVs such as the Toyota Porte and Peugeot 1007 and Renault Kangoo, but are more commonly used in full-sized MPVs like the Toyota Previa, the Citroën C8, the Peugeot 807, the Chrysler Voyager and the Kia Sedona. Their use has increased over the years as MPVs have increased in popularity, because it gives easy access and makes parking in tight spaces possible. The most common type of sliding door, that has a three-point suspension and opens outwards, then runs along the side of the vehicle, was introduced in 1964 by Volkswagen AG as an option on its Type 2 vans.