暗门是与地板、屋顶、剧院舞台或天花板的表面齐平，以滑动或铰接开启的门。暗门的开口也可以是在一个壁上但表面并不齐平。设置暗门的目的是让一般知情的用户访问墙壁、地板或天花板后面的设备，因此暗门又可以被称为舱口或检修门。

A trapdoor is a sliding or hinged door, flush with the surface of a floor, roof, or ceiling, or in the stage of a theatre. A hatch, an opening which may also be in a wall and need not be flush with the surface, is similar; in some cases either name is applicable. A small door in a wall, floor or ceiling used to gain access to equipment is called an access hatch or access door.