A slug or slugline (or “slug line”) is an uppercase line of text with a blank line above and below it.
While the term slugline can be used interchangeably with scene heading, it more often refers to an “intermediary slugline,” which is used to break up and re-focus a longer scene, or to point out an important detail or new element.
Just as Colton has finished wiping most of the peanut butter from his armpits, a
SWARM OF BEES
descends from the air conditioning vent.
Many screenwriters use sluglines in place of SHOT or INSERT. For example, if Sally receives a text message that we read, it may be set off with the slug “ON HER PHONE SCREEN” before the message content, and the slug “BACK TO SCENE” after.
You will often see slugs used as elements of a montage.
This post on writing fight scenes provides more examples of slugs.