Avogadro's Number Definition - Avogadro's Number is the number of atoms or molecules that are contained in a unit mole of a substance. Since this... Avogadro's Number Definition - Avogadro's Number is the number of atoms or molecules that are contained in a unit mole of a substance. Since this value remains constant for all the elements, it is also known as Avogadro constant. It is generally represented as the number of particles that are contained in one mole.