The kilometre, 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 ⁄299 792.458 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 and the United Kingdom where the statute mile is used. A slang term for the kilometre in the military is klick and in common speech involving distance it is often abbreviated as simply a k (kay).