Action line
Also: action · scene description · description
An action line is the prose in a screenplay that describes what the audience sees and hears — movement, behaviour, and visual detail — between the dialogue. It is written in the present tense, set full-width on the page, and kept lean: a screenplay describes only what the camera can capture.
Action lines (also just "action" or "description") carry everything that is not dialogue: a character crossing a room, a door slamming, the weather. They are written in the present tense, third person, and read fast.
The craft of action writing is compression. You describe what is seen and heard, not what is thought or felt internally, and you break long blocks into short paragraphs so the page breathes. White space is a tool, not a waste.