Wikipedia Reference

The stone is a unit of weight and mass. It is part of the Imperial system of weights and measures used in the British Isles, and formerly used in most Commonwealth countries. It is equal to 14 pounds avoirdupois, or 6.35029318 kilograms.

Eight stone make a hundredweight in the Imperial system.

The plural form of stone is correctly stone, though stones is sometimes used, not usually by natives of the British Isles. The abbreviation is st.

History

The stone was historically used for weighing agricultural commodities. Potatoes, for example, were traditionally sold in stone and half-stone (14-pound and 7-pound) quantities. Historically the number of pounds in a stone varied by commodity, and was not the same in all times and places even for one commodity. 

Current use

Although no longer an official unit of measure, the stone remains widely used within the British Isles as a means of expressing human body weight. People in these countries normally describe themselves as weighing, for example, "11 stone 4" (11 stone and 4 pounds), rather than "72 kilogrammes" in most other countries, or "158 pounds" (the conventional way of expressing the same weight in the United States). Its widespread colloquial use may be compared to the persistence in the British Isles of other Imperial units like the foot, the inch, and the mile, despite these having been entirely or partly supplanted by metric measurements in official use (a similar usage persists in Canada, despite that country, unlike the USA, having converted to the metric system in the 1970s).

Outside the British Isles, stone may also be used to express body weight in casual contexts in other Commonwealth countries, particularly Australia and New Zealand.