What type of barrier is primarily defined as a door?

A door is primarily characterized as a moveable barrier used to cover an opening. This definition emphasizes the functional aspect of doors, which serve both to provide access in and out of a room or building and to create a barrier for privacy, security, and temperature control. Unlike a decorative panel, which may serve aesthetic purposes without functionality for providing access, a door actively opens and closes, allowing movement through spaces.

Additionally, a permanent opening refers to an entry that is not designed to be closed, such as an archway or a window, and a fixed partition is a wall that does not have the capability to be moved or opened. Thus, the key distinction lies in the doors' unique ability to be operated, making them a critical component in architectural design and everyday use.