What does the chord line of a wing define?

The chord line of a wing refers to the straight line that connects the leading edge, which is the front part of the wing, to the trailing edge, the rear part of the wing. This line essentially represents the average width of the wing along its length and serves as a critical reference for aerodynamic calculations. Understanding the chord line is fundamental to evaluating the wing's performance in various phases of flight, particularly how it generates lift and interacts with airflow.

In context, while the other options address components and characteristics of the wing, they do not specify what the chord line itself represents. The upper and lower surfaces of the wing are distinct physical elements, the angle of attack pertains to the relationship between the chord line and the oncoming airflow, and the thickness of the wing structure relates to its construction rather than defining the chord line. The focus on the chord length clearly delineates its importance in aerodynamics and wing design.