The ratio between distances on a map and distances in the world
This ratio is usually less than 1:1, because maps typically are smaller than the areas they represent. Notice that a large scale means that a map of given size can represent only a small area, which it usually does in great detail, whereas small scale maps depict large portions of the Earth's surface, with great generalization.
Scales can, and often do, vary from point to point in a map and even can vary with orientation. "The" scale of any map therefore is usually a typical or average scale for that map.