A millimole (mmol) is a unit of measurement used in chemistry to express the amount of a substance, specifically one-thousandth of a mole. A mole is a standard unit in the International System of Units (SI) that quantifies the number of entities, such as atoms or molecules, in a substance, typically equivalent to approximately (6.022 \times 10^{23}) entities. Therefore, one millimole contains about (6.022 \times 10^{20}) entities. Millimoles are commonly used in laboratory settings, particularly in biochemistry and medicine, to measure concentrations of solutes in solutions.