Noun: a device which is used to synchronize the film with the audio track as well as provide a visual indication to the picture editor as to camera reel, day, scene, take, etc. Verb: to identify a sound recording with a spoken description, a tone, etc. May be before the recording (head slate) or after the recording is over (tail slate).
 also known as a clapper. The person associated with this device  is often called the "Clapper/Loader" as they are also responsible for loading the film camera.