While exploring a spooky old house, if you find an opening in the floor covered by a hinged panel, you've discovered a trap door. In mystery novels and movies, trap doors often lead to secret passageways.
You can also call a trap door a hatch, and both terms can be used to describe a hinged or sliding doorway in either a floor or a ceiling, like the trap door you pull open in order to get into an attic. A trap door might also lead to a crawl space or a bomb shelter beneath a house. The earliest use of trap door was recorded in the fourteenth century.