door is a panel made usually of a hard, impermeable, and hard-to-break substance (such as wood or metal), with or without windows, but sometimes consisting of a hard frame into which glass or screens have been fitted, attached to hinges by which it is attached to a frame that constitutes a space for ingress into or egress from a building, room, or vehicle, such that the panel may be moved in various ways (at angles away from the frame, by sliding on a plane parallel to the frame, by folding in angles on a parallel plane, or by spinning along an axis at the center of the frame) to allow or prevent ingress or egress.

In most cases, a door’s interior matches its exterior side but in other cases (e.g., a vehicle door) the two sides are radically different in form to support activities of entering or exiting that differ from one another. Often doors have locking mechanisms to ensure that only the owner or custodian or other persons who have rightful access to a space can open them, and can have knockers or doorbells by which outsiders can announce their presence and summon someone either to open the door for them or give permission to open and enter.

Apart from providing access into and out of a space, doors can have the secondary functions of ensuring privacy by preventing unwanted attention from outsiders, of separating areas with different functions, of allowing light to pass into and out of a space