Plain English Meaning
The drag an aircraft creates simply because of its shape. A flat, blunt shape pushes a lot of air out of the way and leaves messy, swirling air behind it, which slows the aircraft down. A smooth, streamlined shape slips through the air more cleanly and creates less of this kind of drag.
Full Definition
The portion of parasite drag caused by the shape of an object as it moves through the air. Air must separate to flow around the object and rejoin behind it; the wider, blunter, or less streamlined the shape, the more turbulent wake it leaves and the more drag it produces.
Context Anchor
Seen in aerodynamics and aircraft performance discussions, especially when learning why different airplane shapes, landing gear, or flaps change speed and efficiency.