Photographic equipment used to synchronize sound and motion picture; boards held in front of a movie camera are banged together WordNet 3.0 & Open English WordNet

A clapperboard, also known as a dumb slate, clapboard, film clapper, film slate, movie slate, or production slate, is a device used in filmmaking and video production to assist in synchronizing of picture and sound, and to designate and mark the various scenes and takes as they are filmed and audio-recorded. Wikipedia

A film production tool Wikipedia Disambiguation

A device used in film production, having hinged boards that are brought together with a clap, used to synchronize picture and sound at the start of each take of a motion picture or other video production. Wiktionary

Device used in film. Wiktionary (translation)