A door set flat into a floor, ceiling, or roof, rather than upright in a wall. You lift it or it swings down, and underneath is usually a cellar, an attic, a hold in a ship, or a hidden space. The point of a trapdoor is that when it is shut it more or less disappears into the surface — you walk over it without noticing. That hidden quality is why the word turns up so much in stories: the secret way out of a castle, the floor that opens under someone on a stage, the gallows. Stage magicians and theatres genuinely use them, and in old prisons the drop under a hanging man was a trapdoor, which is where some of the word's grimness comes from. In everyday modern life the plainest example is the small square door in a ceiling that you push up to get into the roof space. It also has a technical use in computing, where a trapdoor (more often called a backdoor now) is a hidden way into a system that gets around the normal password check. Written as one word usually, though you will see "trap door" as two, and both are accepted.