mole
SI base unit 

Name  Symbol  Quantity  
mole  mol  amount of substance 

DefinitionThe mole, symbol mol, is the SI base unit of amount of substance. One mole contains exactly 6.022 140 76 × 10^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, N_{A}, when expressed in the unit mol^{−1} and is called the Avogadro number. 
The definition of the mole implies the exact relation N_{A} = 6.022 140 76 × 10^{23} mol^{–1}. Inverting this relation gives an exact expression for the mole in terms of the defining constant N_{A} :
The effect of this definition is that the mole is the amount of substance of a system that contains 6.022 140 76 × 10^{23} specified elementary entities.
The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.