Aspect ratio = wingspan/mean chord or area/chord^2 or wingspan^2/area

All 3 are relevant and all can be derived from the initial version by multiplying the top and bottom by the same term.

Low aspect ratio wings tend to be short and stubby. The wing tip chord is relatively large and allows an increased spillage of air from the lower surface onto the upper surface. The wingtip vortices are consequently increased and this results in increased induced drag. Wings with a high aspect ratio tend to be long and narrow, with a small wing tip chord, so spillage of air around the wingtip is reduced and induced drag decreases.

Consider a rectagular wing and a swept back wing having the same surface area. The sweptback wing will have a lower aspect ratio as the wingspan decreases and the chord increases. Furthermore, the higher the aspect ratio the more aerodynamically efficient the wing.

Note: induced drag is proportional to 1/Aspect Ratio

If a high aspect ratio wing is acted on by an upgust the increase in the angle of attack will result in a larger increase in CL compared to a wing of low aspect ratio. A straight wing will therefore be more susceptible to an upgust than a wing with sweepback.

Typical question: If a wing has a chord of 3m and an area of 30m^2, the aspect ratio will be area/chord^2 = 30/3^2 = 3.33

If a wing has a chord of 5m and an area of 30m^2, the aspect ratio will be area/chord^2 = 30/5^2 = 30/25 = 1.2

This shows that the larger the wing tip chord the lower the aspect ratio and the greater the induced drag.