Plain English Meaning
A smooth, tapered shape that lets the air slide past it cleanly instead of bunching up and creating drag.
Full Definition
A shape or contour designed so that air flows around it smoothly, with minimal disruption, in order to reduce form drag. A streamlined object is rounded at the front and tapered at the rear so the airflow stays attached to the surface rather than breaking away into turbulent eddies.
Context Anchor
Seen in discussions of form drag, aircraft shape, engine covers, landing gear, struts, and other parts exposed to the airflow.
Derivation
From 'stream' (a flow of fluid) plus 'line' (the path it follows). A streamline is literally the line a particle of air traces as it flows. To 'streamline' something is to shape it so the airflow lines stay smooth and unbroken around it.