Definition: A kilometer (symbol: km) is a unit of length in the metric system, equal to 1,000 meters. It is commonly used to measure longer distances such as those between cities or geographical features.

History/origin: The kilometer originated as part of the metric system established during the French Revolution in the late 18th century. It was introduced to provide a decimal-based system of measurement that could easily scale from the base unit of the meter. Over time, the kilometer became widely adopted internationally for measuring large distances.

Current use: The kilometer is extensively used worldwide, especially in countries that have adopted the metric system, for road distances, maps, and geographic measurements. It is the standard unit for expressing distances in transportation, travel, and many scientific disciplines.

Definition: A mile (symbol: mi) is a unit of length commonly used in the imperial and U.S. customary systems. One mile is exactly equal to 1,609.344 meters or 5,280 feet. It is typically used to measure longer distances, such as between towns or along roads.

History/origin: The mile has its roots in the Roman "mille passus," meaning "a thousand paces," which measured approximately 1,480 meters. Over centuries, different countries developed varying definitions of the mile, but the international mile was standardized in 1959 by an agreement between English-speaking countries. This ensured uniformity in the mile’s length as exactly 1,609.344 meters.

Current use: The mile is primarily used in the United States, the United Kingdom, and a few other countries for measuring distances on road signs, in aviation, and for land surveying. While most of the world has adopted the metric system, the mile remains an important unit for everyday use in countries that still employ imperial or customary measurements.