a system of measurement based on absolute units accepted as being fundamental (length, mass, time) and from which other units (force, energy or work, power) are derived; such systems in common use are the foot-pound-second, centimeter-gram-second, and meter-kilogram-second.
The Absolute System of Units, also known as the International System of Units (SI), is a globally recognized system of measurement units that is used in science, industry, and everyday life. It is based on seven fundamental units, known as the SI base units, which are defined by physical standards and have precise and unchanging values.
The seven SI base units are:
- Meter (m): the unit of length, defined as the distance travelled by light in a vacuum in 1/299,792,458 of a second.
- Kilogram (kg): the unit of mass, defined as the mass of the international prototype of the kilogram, which is a cylinder of platinum-iridium alloy kept at the International Bureau of Weights and Measures (BIPM) in France.
- Second (s): the unit of time, defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.
- Ampere (A): the unit of electric current, defined as the constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one meter apart in a vacuum, would produce between these conductors a force equal to 2 × 10−7 newtons per meter of length.
- Kelvin (K): the unit of thermodynamic temperature, defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
- Mole (mol): the unit of amount of substance, defined as the amount of a substance that contains as many elementary entities (atoms, molecules, ions, etc.) as there are atoms in 0.012 kg of carbon-12.
- Candela (cd): the unit of luminous intensity, defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
All other SI units, such as the Newton (unit of force), the Joule (unit of energy), and the Pascal (unit of pressure), are derived from these seven base units. The SI system is regularly reviewed and updated by the International Bureau of Weights and Measures to ensure that it remains current and accurate.