Metric system

A system of measurement is a frame in which physical qualities (such as length, weight, temperature, etc.) can be quantified with numbers. Among many systems of measurement, the metric system refers to ones, each standardises a set of base units and a nomenclature describing relatively large and small quantities using decimal-based multiplicative unit prefixes (such as kilo and milli). The International System of Units (SI) is a good example, and there are other widely accepted systems (less popular than SI) mainly used in specific fields, such as the Gaussian units for electromagnetism.

Though rules governing the metric system have changed over time, the modern definition in the International System of Units prescribes the metric prefixes and seven base units: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd).

An SI derived unit is a named combination of base units, such as the hertz (cycles per second), newton (kg⋅m/s), and tesla (1 kg⋅s⋅A). In the case of degrees Celsius (°C), it is a shifted scale derived from the kelvin (273.15 K is equal to 0 °C while 1 K interval is same to the 1 °C interval).

The SI system derives from the older metre-kilogram-second (MKS) system of units, though the definitions of the base units have evolved over time. Today, all base units are defined by physical constants – not by prototypes in the form of physical objects, as they were in the past.

Other metric system variants include the centimetre–gram–second system of units, the metre–tonne–second system of units, and the gravitational metric system. Each has unaffiliated metric units, and some of these systems are still used in limited contexts. Particular non-SI units such as the litre remain widely used.