It is a specialty door that opens by sliding back and forth on a track instead of swinging out. Because of this feature, a sliding door is regarded as a space saving door. They are also ideal for out of square openings.

A single or double leaf door which opens horizontally by means of overhead trolleys or bottom wheels.

a door with two panels; at least one door panel slides horizontally over the other

a real space saver, as well as being an unusual feature