An acre is a unit for measuring area in the US customary system of measurement.
1 acre = 43,560 ft2
640 acres = 1 mi2

Did you know?

An acre was originally a field that a farmer, using a pair of oxen, could plow in a day. By nightfall, the farmer and the team of oxen would walk about 10 miles to turn an acre of soil.

See also U.S. customary units.