ETYM Old Eng. aker, AS. aecer.
A unit of area (4840 square yards) used in English-speaking countries.
Traditional English land measure equal to 4,840 square yards (4,047 sq m/0.405 ha). Originally meaning a field, it was the size that a yoke of oxen could plow in a day.
As early as Edward I's reign the acre was standardized by statute for official use, although local variations in Ireland, Scotland, and some English counties continued. It may be subdivided into 160 square rods (one square rod equaling 25.29 sq m/30.25 sq yd).