In general "aspect ratio" refers to foils, in any fluid, water or air. The ratio we are interested in is the length of the foil compared to it chord. A long skinny wing, such as found on sailplanes is said to have a high aspect ratio, whereas a fighter jet has stubby low aspect ratio wings. As a rule of thumb, the higher the aspect ratio, the more efficient the wing, i.e. the more lift it can create per unit of drag. This is due the the reduction of wing tip vorticies.