> Originally, an acre was understood as a selion of land sized at forty perches (660 ft, or 1 furlong) long and four perches (66 ft) wide [...]. As a unit of measure, an acre has no prescribed shape; any area of 43,560 square feet is an acre.

Looks like both usages have existed.

"The area of" is important there. If you're going by what wikipedia says, perhaps you could keep reading to the part where it also says "As a unit of measure, an acre has no prescribed shape; any area of 43,560 square feet is an acre."