Mean Aerodynamic Chord (MAC)
From SKYbrary Wiki
The distance between the leading and trailing edge of the wing, measured parallel to the normal airflow over the wing, is known as the chord. If the leading edge and trailing edge are parallel, the chord of the wing is constant along the wing’s length. Most commercial transport airplanes have wings that are both tapered and swept with the result that the width of the wing changes along its entire length. The width of the wing is greatest where it meets the fuselage at the wing root and progressively decreases toward the tip. As a consequence, the chord also changes along the span of the wing. The average length of the chord is known as the mean aerodynamic chord (MAC).
In large aircraft, centre of gravity limitations and the actual centre of gravity are often expressed in terms of percent MAC.
US FAA Aircraft Weight and Balance