A sliding door is a type of door that operates by sliding horizontally on a track. It is typically composed of one or more panels that move smoothly along the track, guided by rollers or bearings. The motion of the door is facilitated by the principles of kinetic and static friction, as well as the force of gravity. The door panel(s) may be made of various materials, such as glass, wood, or metal, and the track may be constructed of materials such as aluminum or stainless steel. Sliding doors are commonly used in homes, offices, and public buildings, due to their space-saving and aesthetic qualities.

Sliding doors have a long history, with evidence of their use dating back to ancient Roman times. The Roman Empire used sliding doors in their homes and public buildings, such as the Colosseum. These doors were typically made of stone or marble and were operated by slaves or pulleys. In the modern era, sliding doors gained popularity in the mid-20th century, particularly in the United States. They were used in mid-century modern architecture, as part of the movement's emphasis on open spaces and streamlined design. Today, sliding doors continue to be a popular choice in contemporary architecture.

Sliding doors are transforming to suit modern living. Frameless designs, offering uninterrupted views, are gaining popularity, especially for patios and balconies. On the tech front, automatic sensors allow doors to slide open or shut upon approach. Multi-panel sliding doors can seamlessly merge indoor and outdoor spaces, creating a vast, open ambiance. With a growing focus on space-saving and fluid design, sliding doors will continue to refine our living spaces, bridging interiors with the outside world effortlessly.