Acrefeet. The international acre is 4046.8564 mò, but the U.S. survey acre is 4046.8726 mò.
In the UK, an acre is defined in the Units of Measurement regulations 1995 http://www.hmso.gov.uk/si/si1995/Uksi_19951804_en_2.htm as 4046.8564224 m².
A square mile has 640 acres, and a square parcel of land ü mile wide has 40 acres.
It was selected as approximately the amount of land tillable by one man behind an ox in one day. Statutory values were enacted in England by acts of Edward I, Edward III, Henry VIII, and George IV; the British "Weights and Measures Act" of 1878 defined it as containing 4840 square yards.