A quantity of land containing 160 square rods of land, in whatever shape. Serg. Land Laws Pa. 185; Cro. Eliz. 476, 665; 6 Coke, 67; Poph. 55; Co. Litt 56. Originally the word “acre” (acer, aher, or Sax. ceqer) was not used as a measure of land, or to signify any determinate quantity of land, but to denote any open ground, (latum quan tumvis agrum,) wide champaign, or field; which is still the meaning of the German acker, derived probably from the same source, and is [>reserved in the names of some places in Eng and, as Castle Acre, South Acre, etc. Burrill.
Law Dictionary – Alternative Legal Definition
measures. A quantity of land containing in length forty perches, and four in breadth, or one hundred and sixty square perches, of whatever shape may be the land. 43,560 square feet or 4,840 square yards.