Chiefly Northeastern U.S. a long, thin board, thicker along one edge than the other, used in covering the outer walls of buildings, being laid horizontally, the thick edge of each board overlapping the thin edge of the board below it.

British. a size of oak board used for making barrel staves and for wainscoting.

of or made of clapboard:

a small board with a hinged stick attached that is clapped clap down at the beginning of the filming of a shot for use later in synchronizing sound and image in the editing of the film.