On a locomotive, the weight on the driving wheels divided by the starting tractive effort. This is an important statistic, because obviously extra power is pointless if there's not enough traction to put it to useful work. A common rule is that a good factor of adhesion exceeds 4, but not by too much. A locomotive with a factor of adhesion below 4 will have a tendency to wheelslip, especially on starting, and will require careful driving and lots of sand in slippery conditions.
Diesel and electric locomotives can work with a much lower factor of adhesion than a steam locomotive because their power is applied smoothly, unlike the latter's pulsed power delivery.
The corollary of the above is that as a locomotive's available tractive effort increases, so must its weight for adequate adhesion. This explains why steam locomotive designers were never that interested in saving weight! Since a steam locomotive must have leading and trailing wheels for stability if it intends to travel at any speed, and these wheels must have some weight on them for good tracking, the steam locomotive's weight increases further. With such an increase in weight, the number of driven axles must also increase; maximum axle load is limited, and thus the tractive effort of a locomotive can be guessed well just by knowing the number of driven wheels and the maximum axle load it was designed for.