Other forms: kilometers
A kilometer is a unit of metric measurement that's about 2/3 as long as a mile. If you sign up for a five kilometer race, you'll be running a course that's a little over three miles long.
Most countries around the world measure geographical distance using kilometers rather than miles — the exceptions are the U.S. and the U.K. The word means "one thousand meters," from Greek roots khilioi, "thousand," and metron, "measure." As you might guess, there are a thousand meters in a single kilometer. If you drive from the U.S. across the Canadian border, expect to see speed limit signs change from miles per hour to kilometers per hour.