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 the metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela.
Note the information on the si below was current up until may 20, 2019 (world metrology day).
Si base units name symbol measure post-2019 formal definition historical origin justification dimension symbol second s time the second, symbol s, is the si unit of time. It is defined by taking the fixed numerical value of the caesium frequency cs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit hz.
Essentials of the SI: Base & derived units
For detailed information on the si base units, see definitions of the si base units and their historical context. Other quantities, called derived quantities, are defined in terms of the seven base quantities via a system of quantity equations. The si derived units for these derived quantities are obtained from these equations and the seven si base units.
It is important to note that the kilogram is the only si unit with a prefix as part of its name and symbol. Because multiple prefixes may not be used, in the case of the kilogram the prefix names of table 5 are used with the unit name gram and the prefix symbols are used with the unit symbol g.
the international system of units (si, abbreviated from the french système international (dunités)) is the modern form of the metric system. It is the only system of measurement with an official status in nearly every country in the world. It comprises a coherent system of units of measurement starting with seven base units, which are the second (the unit of time with the symbol s), metre.