What is the chord of a wing?
the Chord is the line stretching from the leading edge of the wing to the trailing edge parallel to the centerline.


There are different variations to describe the chord of the whole wing.


The Standard Mean Chord
Defined as $\frac S b$ with $S$ the surface area and $b$ the span of the wing. This is not used in aerodynamics.
The Mean Aerodynamic Chord
Defined as $\frac2 S \int_0^{\frac b 2} c(y)^2 dy$ with $y$ the coordinate along the wing and $c(y)$ the chord at $y$.
For swept or delta wings this gives the equivalent chord for a rectangular wing, and is important for placing the Center of Gravity for stability.