Editing Metric system (section)

{{Short description|Decimal-based systems of measurement with 7 base units defined by physical constants}}
{{Use dmy dates|date=December 2018}}
{{Use British English|date=August 2021}}
{{Good article}}
{{For outline|Outline of the metric system}}
[[File:FourMetricInstruments.JPG|thumb|upright=1.2|A [[kilogram]] mass and three metric measuring devices: a [[tape measure]] in [[centimetre]]s, a [[thermometer]] in [[Celsius|degrees Celsius]], and a [[multimeter]] that measures potential in [[volt]]s, current in [[ampere]]s and resistance in [[ohm]]s.]]

The '''metric system''' is a [[system of measurement]] that [[standardization|standardizes]] a set of base units and a [[nomenclature]] for describing relatively large and small quantities via [[decimal]]-based multiplicative [[unit prefix]]es. Though the rules governing the metric system have changed over time, the modern definition, the [[International System of Units]] (SI), defines the [[metric prefix]]es and seven base units: [[metre]] (m), [[kilogram]] (kg), [[second]] (s), [[ampere]] (A), [[kelvin]] (K), [[Mole (unit)|mole]] (mol), and [[candela]] (cd).<ref name=SI2019>{{Cite web|url=https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-EN.pdf |archive-url=https://web.archive.org/web/20190530124226/https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9-EN.pdf |archive-date=2019-05-30 |url-status=live|title=The International System of Units (SI), 9th Edition|date=2019|publisher=Bureau International des Poids et Mesures}}</ref>

An [[SI derived unit]] is a named combination of base units such as [[hertz]] (cycles per second), [[Newton (unit)|newton]] (kg⋅m/s<sup>2</sup>), and [[tesla (unit)|tesla]] (1&nbsp;kg⋅s<sup>−2</sup>⋅A<sup>−1</sup>) and in the case of [[Celsius]] a shifted scale from Kelvin. Certain units have been [[Non-SI units mentioned in the SI#Units officially accepted for use with the SI|officially accepted for use with the SI]]. Some of these are decimalised, like the [[litre]] and [[electronvolt]], and are considered "metric". Others, like the [[astronomical unit]] are not. Ancient non-metric but SI-accepted multiples of time, [[minute]] and [[hour]], are base 60 ([[sexagesimal]]). Similarly, the angular measure [[Degree (angle)|degree]] and submultiples, 
[[arcminute]], and [[arcsecond]], are also sexagesimal and SI-accepted.

The SI system derives from the older [[MKS units|metre, kilogram, second]] (MKS) system of units, though the definition of the base units has evolved over time. Today, all base units are defined by [[physical constant]]s; not by example as 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 [[List of metric units#Other metric units|unaffiliated metric units]]. Some of these systems are still used in limited contexts.