Plain English Meaning
Giving a part a smooth, tapered shape so the air slides around it instead of crashing into it and swirling behind it. Less swirl means less drag.
Full Definition
The shaping of an aircraft component so that air flows smoothly around it, reducing the turbulent wake behind the part and lowering parasite drag. Streamlined shapes are typically rounded at the leading edge and tapered at the trailing edge so the airflow can rejoin cleanly rather than break away into eddies.
Context Anchor
Seen in discussions of parasite drag, aircraft design, fairings, cowlings, landing gear covers, and other shapes that affect how easily the airplane moves through the air.
Derivation
From 'stream' (a flowing line of fluid) plus 'line.' A is the path a single particle of air follows as it moves around an object. To 'streamline' something is to shape it so those lines smooth and unbroken, instead of getting torn apart into .