Units of land area measurement; one acre equals about 4,047 square meters or roughly the size of a football field.
From Old English æcer, related to Latin ager meaning 'field.' Originally it meant the amount of land a pair of oxen could plow in one day, then became standardized into the measurement we use today.
The acre's size was literally based on how much a medieval farmer could actually work—one acre was 'a day's plowing,' so old measurements were really about human work capacity, not abstract numbers like we use today.
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