A kilometer is a unit of distance in the International System of Units (SI) or metric system of measurement. The kilometer is widely used around the world as a measure of distance, with the United States and the United Kingdom clinging to the statute mile. Conversion calculators are readily available for people who need to be able to give distances in both kilometers and miles.
For context for wiseGEEK readers in areas which use other systems of measurement, a kilometer is equal to .62 statute miles, and is sometimes described as being roughly 5/8ths of a mile. This unit of measurement is also roughly equivalent to half a nautical mile. When converting, people often round up or down for convenience; for example, when someone is describing a speed limit as “25 miles per hour” and converts to kilometers, he or she will likely say that the speed limit is “40 kilometers per hour” even though it's technically 40.2336.
The metric system has gone through a number of changes since people first started developing it. The goal of the system was to create a standardized system of measurements which worked on orders of 10. For distance, the base unit is the meter. A meter contains 100 centimeters, smaller units of measurement, 10 decimeters, 1,000 millimeters, and so forth. Conversely, a kilometer contains 1,000 meters, as implied by the name. Kilo means “1,000” and “meter” refers to the base unit of measurement.
As of 1983, the length of a meter is defined as the distance light travels in a vacuum in 1/299,792,458th of a second. This differs from earlier frames of reference which relied on physical objects such as cast metal bars to measure distance. Units in the metric system are given by attaching prefixes to the base unit of measurement. This system is often deemed more convenient and precise by its users, in contrast with imperial units or United States customary units, which use inches, feet, yards, and miles which do not scale neatly up and down as measurements in the SI system do.
People may note that there are different spellings of kilometer. In the United States, the word is spelled “kilometer,” but in other regions, it is seen as “kilometre.” This is one among many spelling variations which reflects the divergence which the English language has experienced. There are also disputes over how the word should be pronounced and where the stresses in the word should be placed.