SI base unit

The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units can be derived. The SI base units and their physical quantities are the metre for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the candela for luminous intensity, and the mole for amount of substance.

The SI base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology.

The names and symbols of SI base units are written in lowercase—for example, the metre (US English: meter) has the symbol m—except the symbols of those named after persons, which are written with an initial capital letter, e.g., the kelvin is named after Lord Kelvin has the symbol K and the ampere after André-Marie Ampère has the symbol A.

Many other units, such as the litre (US English: liter), are formally not part of the SI, but are accepted for use with SI.

2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles."
14th CGPM (1971, Resolution 3; CR, 78)
"In this definition, it is understood that unbound atoms of carbon 12, at rest and in their ground state, are referred to."
(Added by CIPM in 1980)

...
Wikipedia