Avogadro's Number is a fundamental constant in chemistry, defined as 6.022 \times 10^23. It represents the number of atoms, molecules, or particles in one mole of a substance. This number allows scientists to convert between the mass of a substance and the number of particles it contains, making it essential for stoichiometry in chemical reactions. Named after the Italian scientist Amedeo Avogadro, this concept helps in understanding the relationships between different substances in a chemical reaction. By using Avogadro's Number, chemists can accurately measure and predict how substances interact at the molecular level.