furlong (n.)

measure of distance of roughly 660 feet, from Old English furlang, originally the length of a furrow in a common field of 10 acres, from furh "furrow" (see furrow (n.)) + lang "long" (see long (adj.)). The "acre" of the common field being variously measured, the furlong varied but eventually was fixed by custom at 40 rods. Used from 9c. to translate Latin stadium (625 feet), one-eighth of a Roman mile, and so the English word came to be used for "one-eighth of an English mile," though this led to a different measure for the English mile than the Roman one. Furlong being so important in land deed records (where mile hardly figures) it was thought best to redefine the mile rather than the furlong, which was done under Elizabeth I.