A centimeter is a unit of length that is one hundredth of a meter. It’s the equivalent of a quarter inch, or just under three millimeters. Centimeters are used in many areas of science and engineering, including astronomy, geology, archaeology and measuring objects like car engines and medical implants. They’re also sometimes used in everyday life to measure things like the width of a door or the distance between two points.

The kilometer (from the Greek word for “thousand”) is a unit of length, equal to 1000 meters. It was first proposed in 1875 by the French scientist Georges Le Verneur, and was first used in 1880. The SI base unit of measurement is the meter, but since 2006, the kilometer has been recognized as an official unit of measure by the General Conference on Weights and Measures (CGPM), alongside the meter, kilogram and second. The kilometer is not officially defined, but it is based on the distance that light travels in a vacuum in 1/299.792 of a second.