A basic set description located at the top of a script scene, written in all caps, providing information as to whether the scene is interior vs. exterior, day or night, and where it takes place.
Slug lines are unique to the screenplay (as opposed to a stage play). A slug line, also called a master scene heading, occurs at the start of every scene and typically contains three pieces of information: whether the scene is set inside (interior/INT.) or outside (exterior/EXT.), the specific location, and the time of day. Each slug line begins a new scene. In a "shooting script" the slug lines are numbered consecutively for ease of reference.