The international System of Units or SI-system is the coherent system of units adopted and recommended by the General Conference on Weights and Measures (CGPM).

The SI is based at present on the following seven base units:

Base quantity	SI base unit
	Name	Symbol
length	metre	m
mass	kilogram	kg
time	second	s
electric current	ampere	A
thermodynamic temperature	kelvin	K
amount of substance	mole	mol
luminous intensity	candela	cd

Definitions

• The metre is the length of the path travelled by light in vacuum in a time of (1/299 792 458) second (17th CGPM (1983), Res. 1)
• The kilogram is the unit of mass; it equals the mass of the international prototype of the kilogram (1st CGPM (1899))
• The second is the time interval of 9 192 631 770 periods of the radiations corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom (13th CGPM (1967), Res.1)
• The ampère is the constant current that, if kept in two parallel, linear and infinitely long conductors of negligible diameter, placed in vacuum at a distance of 1 meter, executes a power between these two conductors equal to 2 x 10^-7 newton for each meter of length (9th CGPM (1948))
• The kelvin, unit of thermodynamic temperature, is the 1/273.16 th part of the thermodynamic temperature of the triple point of water (13th CGPM (1967), Res.6)
• The mol is the amount of substance of a system that contains as much elementary entities as the number of atoms in 0.012 kilogram of carbon 12 (14th CGPM (1971), Res.3)
  When using the mol as a unit one has to specify the elementary entities; these can be atoms, molecules, ions, electrons, other particles or certain groups of such particles.
• The candela is the luminous intensity in a specified direction of a source that emits monochromatic radiation with a frequency of 540 x 10¹² hertz and of which the intensity of radiation in that direction is 1/683 watt per steradian (16th CGPM (1979), Res.3)