A door is a panel or barrier, usually hinged or sliding, that is used to cover an opening in a wall or partition going into a building or space. A door can be opened to give access and closed more or less securely. The term door is also applied to the opening itself, more properly known as the doorway.

Doors are nearly universal in buildings of all kinds, allowing passage between the inside and outside, and between internal rooms. When open, they admit ventilation and light.

The purpose of a door closure is primarily to give occupants of a space privacy and security by regulating access. For this purpose doors are equipped with a variety of fittings ranging from simple latches to locks.

The door is used to control the physical atmosphere within a space by enclosing it, excluding air drafts, so that interiors may be more effectively heated or cooled. Doors are significant in preventing the spread of fire.

Doors also have an aesthetic role in creating an impression of what lies beyond. They are also used to screen areas of a building for aesthetic purposes, keeping formal and utility areas separate. They act as a barrier to noise.

Doors are often symbolically endowed with ritual purposes, and the guarding or receiving of the keys to a door, or being granted access to a door can have special significance. Similarly, doors and doorways frequently appear in metaphorical or allegorical situations, literature and the arts, often as a portent of change.

When framed in wood for snug fitting of a door, the doorway consists of two vertical jambs on either side, a lintel or head jamb at the top, and perhaps a threshold at the bottom. When a door has more than one movable section, one of the sections may be called a leaf.

See door furniture for a discussion of attachments to doors such as doorhandles and doorknobs. Doors are also found in cupboards and other furniture, cages, and vehicles.

Types of doors

A door may slide along tracks, pivot on hinges or fold.

The door may also slide between two wall panels (pocket door).

In the case of rotation, the axis is usually vertical, but e.g. for garage doors often horizontal, above the door opening. Sometimes the axis of rotation is, with a special construction, not in the plane of the door, on the other side than that in which the door opens, to reduce the space required on the side to which the door opens. This is sometimes the case in a train, for the door to the toilet, opening inward.