International System of Units
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The International System of Units, or the SI units (shortened from the French Système international d'unités) is the globally-adopted system of units of measurement. It is the modern iteration of the metric system. It superseded all prior systems including CGS and MKS, although in certain fields, including radiation physics, some legacy units linger, especially in the USA, e.g. rad.
Seven base units are defined:
- second (s) (time)
- meter (m) (distance)
- kilogram (kg) (mass)
- ampere (A) (current)
- kelvin (K) (temperature)
- mole (mol) (amount of substance)
- candela (cd) (luminous intensity)
All other units can be derived from these base units and are known as the derived SI units, e.g. hertz.