Short answers, pulled from the story.
An acre is a unit of land area in the British imperial and United States customary systems. It equals 4,840 square yards, or 43,560 square feet, or about 4,047 square metres, which is roughly 40 percent of a hectare. It has no prescribed shape, so any area of 43,560 square feet is an acre.
The word acre is first attested in 1006 in a text from Fecamp in Normandy, meaning an agrarian measure. It descends from the old Scandinavian akr, meaning cultivated field or ploughed land, and is related to the German Acker, the Latin ager, the Sanskrit ajr, and the Greek agros. In the Middle Ages an acre was conceived as the land one man and a team of eight oxen could plough in a day.
The international acre measures about 4,046.856 square metres, while the US survey acre measures about 4,046.872 square metres, a difference of only about four parts per million. The survey acre is based on an inch set by the Mendenhall Order of 1893, defining one metre as 39.37 inches exactly. The gap is about 0.016 square metres, roughly a quarter of an A4 or US letter sheet.
The acre lost its standing as a primary unit for trade in the United Kingdom from the 1st of October 1995, after a 1994 amendment to the Weights and Measures Act replaced it with the hectare. Land registration kept an exemption until HM Land Registry ended it in 2010. The acre remains lawful indefinitely as a supplementary unit beside the hectare.
There are 640 acres in one square mile. A quarter of a square mile measures 880 yards on each side and contains 160 acres, and a quarter of that contains 40 acres, the parcel behind the American farm phrase the back 40.
A builder's acre is 40,000 square feet, or 200 by 200 feet, used in US real estate development to simplify the math and for marketing. It is nearly 10 percent smaller than a survey acre, and that discrepancy has led to lawsuits alleging misrepresentation.