SI

SI indicates the International System of Units, a system of the metric units standardized through cooperative international effort. SI can be considered a standardized version of a pre-existing system of units termed MKS (for "meter, kilogram, second"). SI includes refined definitions for a basic set of units (meter, kilogram, second, kelvin, mole, and candela) and an extensive set of more specialized units derived from these. The general trend of these refinements is to redefine them in terms of physical constants, using constants that are very-reliably measured to high accuracy: when such measurements become more reliable and accurate than other means of defining the unit (such as creating and preserving a meter-length rod), then reason suggests using those physical constants to define the unit. The common metric units (e.g., meter, kilogram) predate the system and are also used in other similar metric-based systems.

Some non-SI units remain in scientific use, especially for electromagnetic measurement, such as for magnetic flux density: in addition to the SI unit, the tesla, a non-SI (CGS) unit, the gauss, is still in use. Fields of science and technology sometimes coin their own non-standard units, at a convenient order-of-magnitude and/or for a relevant quantity: in astrophysics, among them are the foe, a unit of energy, and the jansky, a unit of spectral flux density.