A furlong is a unit of measure in the UK and the US, which is not seen often in modern times. It translates to an eighth of a mile, 660 feet, or 220 yards (.20 km or 201.17 m). Thus if you were traveling at a speed of 10 mph (16.09 kph), you’d essentially travel 80 furlongs per hour.
Furlong is closely related to the Greek and Roman measurement of stade, which is a little smaller. The stade is 625 feet (190.5 m). Early English translations of Greek text in the Bible complicated the matter of measurement by substituting furlong for stade, despite the fact that this was inaccurate. Like furlongs though, the stade constituted an eighth of the Roman mile.
After Roman control of Europe fell, the British retained much of these original units of measure, and published specific measurements by the 1300s. Furlongs were especially important, since they tended to be the measurement of a row or furrow in an acre. The British acre was essentially one furlong long and one chain, 22 yards (20.12 m), wide. The American football field is about half a furlong in length.
Furlong, in word origin is a portmanteau word, a combination of the words furrow (fuhr) and lang, meaning long. The word is derived from Anglo-Saxon, and survived through the transition into Middle and then Modern English. Generally, the most practical use of the word day as a measurement is when discussing distances in horse races. Distances of a race are still measured as both miles and furlongs.
Since the UK has adopted the metric system, and the US has tried also to adopt it, you’re likely to see much less use of the furlong as a unit of measure in the future. In 1985 the UK passed the Weights and Measures Act, which standardized the use of metric measurements. The metric system is in many ways more practical to use, since it is constructed on a base 10 system, making conversions far easier.
Conversions are much more difficult when units of measure are somewhat random, as with US customary units. A foot is 12 inches, a yard is three feet, and furlong is an eighth of a mile. You can’t always depend on the same number with measurements like the mile or foot to figure out how each unit fits with the others.