Millimeters are a unit of length measurement and a unit of rainfall. The English abbreviation is mm. 1 mm is equivalent to one thousandth of a meter, one hundredth of a decimeter, and one tenth of a centimeter.
One of the many metric prefixes is the milli-prefix. It is one thousandth of the base unit. In this case, it is the meter. Over time, the definition of a meter has changed. The current definition is based on how far one can travel at the speed of light over a certain time period. However, the relationship between the millimeter and the meter remains constant. The length of a prototype bar meter was the basis for the definition of the meter. The definition of a second has changed in 2019, and the meter was re-defined.