SI indicates the International System of Units, a current, widely-accepted system of the metric units, standardized internationally and overseen by international organizations. The system 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 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 common 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.