A door is a moving structure used to block off, and allow access to, an entrance to or within an enclosed space, such as a building or vehicle. Similar exterior structures are called gates. Typically, doors have 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 interior 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. Doors normally consist of a panel that swings on hinges or that slides or spins inside of a space.

Rational Dynamic Object Oriented Requirements System (DOORS) (formerly Telelogic DOORS) is a requirement management tool. It is a client–server application, with a Windows-only client and servers for Linux, Windows, and Solaris. There is also a web client, DOORS Web Access.