Definition: A kilometer (km) is a unit of length in the metric system, equal to one thousand meters. It is a decimal multiple of the meter, the SI base unit of length.

History/origin: The kilometer is derived from the meter, which was defined in the late 18th century during the French Revolution. The prefix 'kilo-' denotes one thousand.

Current use: Kilometers are commonly used for measuring longer distances, such as the distance between cities, the length of a road, or the distance covered in a race. They are the standard unit of distance in many countries.