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 1⁄ 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. There are two pronunciations for the
word: The former pronunciation follows the general pattern in
English whereby metric units of measurement are pronounced with
the stress on the first syllable, and preserves the
pronunciation of metre. It is generally preferred by the BBC,
while most scientists use the pronunciation with stress on the
second syllable. The latter pronunciation, which follows the
stress pattern used for the names of measuring instruments (such
as micrometer, barometer, thermometer, tachometer and
speedometer), is in common usage as well. Kingsl