the system of measurement that uses the metre, the kilogram and the litre as basic units Culture The metric system is used in most countries in the European Union and is replacing the imperial system in Britain. The metric system is sometimes also called the International System or Système International and the units of measurement are called SI units. Measurements are each derived from a single base unit, for example the metre for length and the kilogram for mass (= weight). Other units of measurement are 10, 100, 1 000, etc. times smaller or larger than these. Some metric units of measure are spelt differently in British and American English: words ending in -metre and -litre in British English end in -meter and -liter in American English.