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.
Plain English
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.
Context Anchor
Seen in aerodynamics and aircraft performance discussions, especially when learning why different airplane shapes, landing gear, or flaps change speed and efficiency.
Derivation
Form' here means shape or outline, from the Latin 'forma' meaning shape or figure. So 'form drag' is literally 'shape drag' -- the drag that comes from the form of the object itself, not from its surface roughness or other factors.
Why Pilots Care
Form drag rises quickly with speed and poor streamlining, directly lowering cruise efficiency and increasing fuel burn.
Analogy
Hold your flat palm out a car window at speed and you feel a strong push. Turn your hand sideways like a knife and the push almost disappears. Same hand, same speed -- different form, different drag.
Intuition Check
Do not read “form” as a paperwork form here. In form drag, “form” means the physical shape of the airplane or part moving through the air.
Example Sentence 1
Retractable landing gear reduces form drag in cruise by tucking the wheels and struts inside the streamlined fuselage.
Example Sentence 2
Fairings installed on the landing gear reduced form drag and improved fuel economy during cross-country flights.