The recommended practical system of units of measurement is the International System of Units (Système International d'Unités), with the international abbreviation SI.
The SI was previously defined in terms of seven base units and derived units defined as products of powers of the base units. The seven base units were chosen for historical reasons, and were, by convention, regarded as dimensionally independent. They are
After decades of ground-breaking laboratory works, the world’s scientific and technical community, in a landmark and historic decision taken in the General Conference on Weights and Measures (CGPM), Versailles, France organised by International Bureau of Weights and Measures (BIPM) on 16 November 2018, has unanimously adopted the resolution to redefine four of the seven base units, the kilogram (SI unit of weight), Kelvin (SI unit of temperature), mole (SI unit of amount of substance), and ampere (SI unit of current).As per the revised SI,
The seven defining constants of the SI are:
Although the size of these units will not change (a kilogram will still be a kilogram), the four redefined units will join the second, the metre and the candela to ensure that the set of SI base units will continue to be both stable and useful. The revised SI will maintain its relevance by facilitating technical innovations.
The new SI is being implemented worldwide from 20th May 2019.
Source : BIPM