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.
Plain English
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.
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 streamline 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 stay smooth and unbroken, instead of getting torn apart into turbulence.
Why Pilots Care
Lower drag means higher cruise speeds, reduced fuel burn, and better climb performance without added power.
Analogy
Think of holding your hand flat out the car window versus turning it edge-on. Same hand, same speed, very different push. Streamlining is shaping the part so the air meets it edge-on as much as possible.
Intuition Check
Streamlining does not mean making a procedure simpler here. It means shaping the aircraft so air can pass around it more smoothly.
Example Sentence 1
Wheel pants are added to fixed-gear airplanes to streamline the landing gear and reduce parasite drag in cruise.
Example Sentence 2
Streamlining the wing struts allowed the older biplane to reach a higher top speed.