Avogadro's number (Avogadro constant, abbreviated NA) is a large number aimed at representing the number of nucleons (protons and neutrons) in a gram of substance. No single number can do this with perfect precision because the mass of atoms and molecules do not precisely match (the sum of) their mass numbers. Avogadro's number is chosen to be a realistic representative, now having the standard definition of 6.02214076 × 1023. A mole (mol) of a substance is defined as that many molecules. The number is convenient for chemists, who can relate grams to elements and molecular formulae, and can be convenient in calculations for allowing smaller numbers to be used than particle counts.
Earlier in its history, Avogadro's number and the mole were pinned to the standard gram along with some selected element isotope, but (like the AU) it is now considered more useful to have a stable, precise number not subject to future change due to improving measurement.