A door is a movable barrier that opens and closes to control entry to a room, building, or vehicle. Doors let us move between spaces while also giving us privacy and security when we need it.

Most doors swing on hinges, but some slide along tracks, fold like accordions, or roll upward like garage doors. Ancient civilizations built massive stone doors that pivoted on central pins, while modern buildings might have automatic doors that sense your approach and open for you.

The word also appears in many useful phrases. When someone says an opportunity is knocking at your door, they mean a good chance has arrived. If you say someone opened doors for you, they helped create opportunities. When you hear “don't let the door hit you on the way out,” someone is rudely suggesting you should leave quickly. And if something happens next door, it's happening in the adjacent building or room.

People sometimes describe other people as doormats if they let others walk all over them, or as door-to-door salespeople if they visit houses trying to sell things. A doorway is the opening that a door fills, and a doorstep is the area right in front of a door where you might find packages delivered or guests waiting to be welcomed inside.