Millimeter is a unit of length that is one thousandth of a meter. It's often used in measurements to make them more understandable to the average person. Millimeters are also commonly used to describe the sizes and dimensions of things. Millimeter is the smallest unit of measurement in the International System of Units (SI). It is also called a subdivision of a centimeter. One millimeter is about the width of a human hair.

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.