A "mol" is a unit of measurement in chemistry that represents a specific quantity of particles, such as atoms or molecules. One mole is defined as exactly 6.022 \times 10^23 particles, a number known as Avogadro's number. This unit helps chemists quantify substances in a way that relates to their atomic or molecular scale. Moles are essential for stoichiometry, which is the calculation of reactants and products in chemical reactions. By using moles, scientists can predict how much of each substance is needed or produced in a reaction, making it easier to understand and manipulate chemical processes.