SI base unit

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The international system (SI) of units defines seven SI base units: physical units defined by an operational definition.

All other physical units can be derived from these base units: these are known as SI derived units. Derivation is by dimensional analysis. SI prefixes are used to abbreviate long numbers.

The following are the base units from which all others are derived. They are dimensionally independent, with the exception of the metre and candela. The candela was formerly a fundamental unit but has been redefined in terms of the other SI base units. The metre also was a fundamental unit, but has since been redefined in terms of the second. They are still considered "base units" for historical reasons, but are in fact dependent on the other units for their definition.

SI base units
Name Symbol Measure Definition Historical Origin/Justification
metre or meter m length The unit of length is equal to the length of the path travelled by light in a vacuum during the time interval of $\frac{1}{299,792,458}$ of a second. Defined by: 17th CGPM (1983) Resolution 1, CR 97 $\frac{1}{10,000,000}$ of the distance from the earth's equator to the North Pole measured through Paris.
kilogram kg mass The unit of mass is equal to the mass of the international prototype kilogram (a platinum-iridium cylinder) kept at the Bureau International des Poids et Mesures (BIPM), Sèvres, Paris (1st CGPM (1889), CR 34-38). Note that the kilogram is the only base unit with a prefix. See the kilogram article for an alternative definition. The mass of one litre of water. Kilogram was originally named "grave" and symbolized G. The gram is defined as a derived unit, equal to $\frac{1}{1000}$ of a kilogram; prefixes such as mega are applied to the gram, not the kg; e.g. Gg, not Mkg. It is also the only unit still defined by a physical prototype instead of a measurable natural phenomenon.
second s time The unit of time is the duration of exactly 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom at a temperature of 0 K. Defined by: 13th CGPM (1967-1968) Resolution 1, CR 103 $\frac{1}{86400(24*60*60)}$ of the day
ampere A electrical current The unit of electrical current is the constant current which, if maintained in two straight parallel conductors, of infinite length and negligible cross-section, placed 1 metre apart in a vacuum, would produce a force between these conductors equal to $2*10^{-7}$ newtons per metre of length. Defined by: 9th CGPM (1948) Resolution 7, CR 70
kelvin K thermodynamic temperature The unit of thermodynamic temperature (or absolute temperature) is the fraction $\frac{1}{273.16}$ (exactly) of the thermodynamic temperature at the triple point of water. Defined by: 13th CGPM (1967) Resolution 4, CR 104 $\frac{1}{100}$ of the difference between the boiling and freezing points of water
mole mol quantity of matter (mass/mass) A mole is the quantity of substance that contains the same number of elementary entities (atoms, molecules, ions, electrons or particles, depending on the substance) as there are atoms in 0.012 kilograms of pure carbon-12; this number (NA) is approximately equal to $6.0221415(10)*10^{23}$ mol-1 (2002 CODATA). one gram per atomic mass unit
candela cd luminous intensity The unit of luminous intensity is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency $540*10^{12}$ hertz and that has a radiant intensity in that direction of $\frac{1}{683}$ watt per steradian. Defined by: 16th CGPM (1979) Resolution 3, CR 100 the candlepower