acre

   1. (used as a unit of length)

   various historical references mention "acre" as a unit of length, but there has never been any official definition of such a unit. In older works, especially in Britain, an acre of length is a furlong and an acre of breadth is 4 rods, since those were the historic dimensions of an acre. The acre is also the area of a square about 208.710 feet (roughly 208 feet 8.5 inches or 63.615 meters) on a side, and sometimes, mostly in the U.S. and Canada, this length was called an acre or the side of an acre. In contrast, the original area unit was sometimes called the square acre. All these usages are obsolete.

   2. (ac or A)

   a unit of area used for measuring real estate in English-speaking countries. "Acre," an Old English word meaning a field, is derived from the Latin ager and Greek agros, also meaning a field. The acre was originally defined as the area that could be plowed in a day by a yoke of oxen. It was in use in England at least as early as the eighth century, and by the end of the ninth century it was generally understood to be the area of a field one furlong (40 rods or 10 chains) long by 4 rods¹ (or 1 chain) wide. Thus an acre is 10 square chains, 160 square rods, 43 560 square feet or 4840 square yards. There are exactly 640 acres in a square mile. In metric countries the unit corresponding to the acre is the hectare, which is 10,000 square meters (the area of a square 100 meters on each side). One acre is equal to 0.404 687 3 hectare. Among traditional European land area units, the acre is typical in being defined as a day's work but unusual in not being visualized as the area of a square. Similar units include the French journal, north German and Dutch morgen, south German and Swiss juchart, Austrian joch, and Czech jitro.