A "mol" is a unit in chemistry that measures the amount of substance. It is defined as containing exactly 6.022 x 10^23 particles, which can be atoms, molecules, or ions. This number is known as Avogadro's number and helps scientists quantify and compare different substances in a consistent way. Using moles allows chemists to convert between mass and the number of particles easily. For example, one mole of carbon weighs about 12 grams. This concept is essential for balancing chemical equations and conducting reactions, making it a fundamental part of stoichiometry.