Centimeter. Definition: A centimeter (symbol: cm) is a unit of length in the International System of Units (SI), the current form of the metric system. It is defined as 1/100 meters. History/origin: A centimeter is based on the SI unit meter, and as the prefix "centi" indicates, is equal to one hundredth of a meter. Metric prefixes range from factors of 10-18 to 10 18 …