Most wings change their profile over their width (or span) so it's common practice to instead talk about mean aerodynamic chord, or MAC. The MAC is somewhat more complex to calculate, because most wings vary in area over the span, growing thinner towards the outer tips. This means that more lift is generated on the thicker inner portions, and the MAC moves moves the point to measure the chord to take this into account. (If a wing was rectangular, rather than tapering or swept, then the chord would simply be the length of the wing in the direction of airflow.)
Standard mean chord (SMC) is defined as wing area divided by wing span.
,
where is the wing area and is the span of the wing.
Mean aerodynamic chord (MAC) is defined as
The ratio of the chord of a wing to its width (or span) is known as the aspect ratio an important indicator of the lift-induced drag the wing will create. In general planes with higher aspect ratios - wide skinny wings - will have less drag. This is why gliders have such long wings.
See also: Lift (force)