Plain English Meaning

Wings are usually tapered or swept, so they don't have one obvious 'width' from front to back. The MAC is a calculated average wing width that engineers and pilots use as a standard reference line. Where the airplane balances (its center of gravity) is described as a percentage along this line.

Full Definition

The mean aerodynamic chord (MAC) is a single reference chord line used to represent the aerodynamic behavior of an entire wing. It is the chord of an imaginary rectangular wing that would produce the same lifting force and pitching moment as the actual wing. The MAC is used as the reference line against which the airplane's center of gravity (CG) position is expressed, typically as a percentage of MAC.