The acre is a unit used to measure land area in both the imperial and US customary systems. It is traditionally defined as an area of 43,560 square feet (approximately 4,047 m2, or about 40% of a hectare).

The word ‘acre’ itself is derived from the Old English word ‘æcer’ (originally meaning ‘open field’) which in turn came from the Latin word ‘ager’, which also meant ‘field’. The word is fitting because the acre was originally created to be equivalent to the area of land in a field that could be plowed by a team of oxen, in one day.

While the amount of land, or acreage, that modern humans can plow in one day has exponentially increased, the name stuck for this particular moderately sized piece of land as a unit of measurement.

More Info: en.m.wikipedia.org