Plain English Meaning

The control surface is sitting flush with the air flowing past it, so the air slides over it smoothly and the surface isn't pushing the airplane in any direction.

Full Definition

The position of a flight control surface (such as the rudder or elevator) when it is aligned with the surrounding airflow, producing minimum aerodynamic drag and exerting no control force on the airplane.

Context Anchor

Seen in discussions of how ailerons, elevators, and rudders create movement when they are moved away from their neutral position.

Derivation

From 'streamline' — the smooth, unbroken path that air or follows around an object. A surface in its 'streamlined position' lies along that natural flow line, so the air isn't disturbed.