What is meant by a 'mole' of a substance?

The mole is the unit of measurement in the International System of Units (SI) for the amount of substance. It is defined as the amount of a chemical substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons, or photons) as the Avogadro constant($N_A$), which has a value of $6.022\times {10}^{23}mo{l}^{-1}$.