A door is an opening/closing structure used to block off an entrance, typically consisting of an interior side that faces the inside of a space and an exterior side that faces the outside of that space. While in some cases the exterior side of a door may match its exterior side, in other cases there are sharp contrasts between the two sides, such as in the case of the vehicle door. In addition, doors typically consist of a panel that swings on hinges or that slides or spins inside of a space.

When open, doors admit people, animals, ventilation, and light. The door is used to control the physical atmosphere within a space by enclosing the air drafts, so that interiors may be more effectively heated or cooled. Doors are significant in preventing the spread of fire. They act as a barrier to noise. Many doors are equipped with locking mechanisms to allow entrance to certain people and keep out others.

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