The "Amount of Substance" is a fundamental concept in chemistry that quantifies the number of entities, such as atoms, molecules, or ions, in a sample. It is measured in moles, where one mole corresponds to approximately 6.022 x 10^23 entities, known as Avogadro's number. This measurement helps chemists understand and predict the behavior of substances in reactions. In practical terms, the amount of substance allows scientists to calculate concentrations, determine stoichiometry in chemical reactions, and convert between mass and number of particles. It is essential for accurate measurements and formulations in various fields, including pharmaceuticals and materials science.