What does the term "airfoil" refer to?

The term "airfoil" refers specifically to the shape of a wing or blade (such as a propeller) that is designed to generate lift when air flows over it. The design of an airfoil is crucial in aerodynamics because it affects the lift force produced as well as the overall performance of an aircraft or any other flying vehicle.

An airfoil typically has a specific cross-sectional shape, with a curved upper surface and a flatter lower surface, which creates a pressure difference between the top and bottom surfaces when air passes over it. This difference in pressure results in lift, allowing an aircraft to rise and sustain flight.

Other options presented in the question do not accurately define what an airfoil is; instead, they relate to other aspects of aviation. For instance, a device used to control altitude is related to control surfaces, while a navigation instrument would pertain to tools used for determining position or direction. The location of landing gear is also unrelated, as it concerns the structure and support of the aircraft when on the ground. Understanding the role of an airfoil is fundamental in grasping the principles of flight and aircraft design.