What is the Foot ?

Source: en.citizendium.org

" The foot (plural foot or feet; symbol or abbreviation: ft or the single prime ′ ) is a unit of length in the Imperial and U.S. customary systems of measurement. It is equal to 12 inches or one third of a yard.
It is used primarily in the United States and, although it has been officially superseded by units of the metric system, it is still commonly used in the United Kingdom.
Historically the "foot" was a part of many local systems of units, including the Greek, Roman, Chinese, French, and English systems (...) "