What is Acre?

Legal Definition
The acre is a unit of land area used in the imperial and US customary systems. It is defined as the area of 1 chain by 1 furlong (66 by 660 feet), which is exactly equal to 1640 of a square mile, 43,560 square feet, approximately 4,047 m2, or about 40% of a hectare.

The acre is commonly used in Antigua and Barbuda Australia, American Samoa, the Bahamas, Belize, the British Virgin Islands, the Cayman Islands, Canada, Dominica, the Falkland Islands, Grenada, Ghana, Guam, the Northern Mariana Islands, India, Sri Lanka, Bangladesh, Nepal, Ireland, Jamaica, Montserrat, Myanmar, Pakistan, Samoa, St. Lucia, St. Helena, St. Kitts and Nevis, St. Vincent and the Grenadines, Turks and Caicos, the United Kingdom, the United States and the US Virgin Islands.

The international symbol of the acre is ac. The most commonly used acre today is the international acre. In the United States both the international acre and the US survey acre are in use, but differ by only two parts per million, see below. The most common use of the acre is to measure tracts of land. One international acre is defined as exactly 4,046.8564224 square metres.

An acre was defined in the Middle Ages, being the amount of land that could be ploughed in one day with a yoke of oxen.
-- Wikipedia
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. Serg. Land Laws of Penn., 185. See Cro. Eliz. 476, 665; 6 Co. 67; Poph. 55; Co. Litt. 5, b, and note 22.
-- Bouviers Law Dictionary
Legal Definition
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. 5b. Originally the word "acre" (acer, aker, or Sax. oeccr) was not used as a measure of land, or to signify any determinate quantity of land, but to denote any open ground, (latum quantumvis 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 Bng-and, as Castle Acre, South Acre, etc. Burrill.
-- Black's Law Dictionary