Mini-slug
Also: secondary slug · sub-slug
A mini-slug is a short, partial scene heading used to move the eye within a single location without resetting the scene. Writers use them — a single word like "KITCHEN" or "ON THE PHONE" — to control pace and direct attention inside a master scene, without the overhead of a full slugline.
A mini-slug (or secondary heading) is a writer's pacing tool. Inside a long master scene, dropping a one-line uppercase fragment — "THE DOOR.", "CLOSE ON THE LETTER." — snaps the reader to a new beat without breaking the scene into a new numbered scene.
Because a mini-slug does not start a new scene, it does not carry an INT./EXT. or a time of day. Overuse turns into shot-listing, which is the director's job, so working writers keep them sparse and load-bearing.