A base unit is a fundamental measurement that is defined by a readily reproducible reference standard. The seven base units of the International System of Units (SI) are:

- Meter (m): the unit of length, defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second.

- Kilogram (kg): the unit of mass, defined as the mass of the international prototype kilogram, a cylinder of platinum-iridium alloy kept at the International Bureau of Weights and Measures (BIPM) in Sèvres, France.

- Second (s): the unit of time, defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two energy levels of the cesium-133 atom.

- Ampere (A): the unit of electric current, defined as the flow of electric charge of one coulomb per second.

- Kelvin (K): the unit of thermodynamic temperature, defined as 1/273.16 of the thermodynamic temperature of the triple point of water.

- Mole (mol): the unit of amount of substance, defined as the amount of substance of a system that contains exactly 6.02214076×10^23 elementary entities.

- Candela (cd): the unit of luminous intensity, defined as the luminous intensity of a light source that emits monochromatic radiation of frequency 540×10^12 hertz with a radiant intensity of 1/683 watt per steradian.

These base units are used to define all other units in the SI system, which is the most widely used system of measurement in the world.