The chordline is defined as:

The chordline of an airfoil is indeed best defined as an infinitely long straight line drawn through the leading and trailing edges of the airfoil. This definition captures the essential characteristics of the chordline as it represents the straight geometric line that establishes the basic profile of the wing.

In aerodynamic terms, the chordline is an important reference because it serves as the baseline from which other measurements and calculations are made, such as the calculation of angles of attack or aerodynamic forces acting on the wing. It allows for a standardized way to assess the aerodynamic performance of various airfoil shapes, regardless of their size or specific design features.

The other definitions do not accurately encapsulate what the chordline represents. For example, while the wings center line refers to a different aspect of the wing's structure, the measure of the length of the wing is more related to the wingspan rather than the chordline itself. The average distance from the wing root to the wingtip relates more to the overall geometry of the wing rather than defining the chordline.