Merriam-Webster describes it this way: 
• a movable piece of wood, glass, or metal that swings or slides open and shut so that people can enter or leave a room, building, vehicle, etc.
• a part of an object (such as piece of furniture or an appliance) that swings or slides open and shut
• the opening for a door : the entrance to a room or building

The Ada Icon describes it this way:
• Ada elementary students decorated school doors for the holidays. Here's a sampling of what Monty Siekerman photographed this week during a walk through the school.