A short ton most commonly references its weight, the force exerted by its mass at standard gravity (i.e., short ton-force). One short ton contains 2,000 pound-mass (lb), which corresponds to a weight at one standard gravity of 2,000 pound-force (lbf).
In the United States, a short ton is usually known simply as a "ton", without distinguishing it from the tonne (1,000 kilograms or 2,204.62262 pounds), known there as the "metric ton", or the long ton also known as the "imperial ton" (2,240 pounds or 1,016.0469088 kilograms). There are, however, some U.S. applications where unspecified tons normally mean long tons (for example, naval ships) or metric tons (world grain production figures).
Both the long and short ton are defined as 20 hundredweights, but a hundredweight is 100 pounds (45.359237 kg) in the US system (short or net hundredweight) and 112 pounds (50.802345 kg) in the imperial system (long or gross hundredweight).
The United Kingdom used to use the imperial ton, known as the long ton in the United States. The use of the imperial ton ceased under the Weights and Measures Act 1985. The measure used since then is the tonne (metric ton) and the word "ton", if used, is taken to refer to an imperial or long ton. Most Commonwealth countries followed British practice with the exception of Canada, which continued to use short tons as well as long tons but now predominantly uses metric tons (tonnes).
20 hundredweights = 1 ton