The kilometre (American spelling: kilometer), symbol km is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second. It is the conventionally used measurement unit for expressing distances between geographical places in most of the world; notable exceptions are the United States, where the statute mile is used, and the United Kingdom. Although the United Kingdom Ordnance Survey adopted gri… (More on Kilometer)