CAVITY

[noun] [C] - a hole, or an empty space between two surfacesThe gold was hidden in a secret cavity in the chimney behind a loose brick.My dentist said I have a cavity (= holes in a tooth) which will have to be treated.A cavity wall is a wall of a building formed from two walls with a space, usually for air, between them. It is made in this way to keep out cold and wetness.