a door: the entrance to a house, room, or enclosure, and the panel that swings to open or close it
The word names an everyday thing: the door of a house, a room, a tomb, or a prison, and by extension the opening it guards. Most of its appearances describe exactly that plain, physical object. A door is shut for private prayer, a stone is rolled against the door of a grave, frightened followers huddle behind locked doors, and an angel opens the doors of a jail.
Because a door is the place where you get in or are kept out, the word turns figurative in the same natural way the English word does. An open door becomes a chance to act, above all a chance to spread the message. To say an event is at the door is to say it is about to arrive. Jesus can even call himself the door, meaning that he is the one way in to safety and life. And the door of the wedding feast or of the kingdom can be shut, closing out those who come too late.
Underneath every use lies a single idea: entry. Whether the door is made of wood and hinges or stands as a picture of opportunity, nearness, or salvation, it marks the line between outside and inside, and the moment at which that line is crossed or closed.