A millimeter (mm) is a unit of length in the metric system, equal to one-thousandth of a meter. It is commonly used to measure small distances or dimensions, such as the thickness of a credit card or the diameter of a pencil lead. The metric system is widely used around the world, making millimeters a standard unit for various applications in science, engineering, and everyday life. Millimeters are often used in fields like engineering, manufacturing, and medicine to ensure precision. For example, a caliper can measure objects in millimeters, allowing for accurate specifications in product design. Understanding millimeters helps in converting measurements to other units, such as centimeters or inches.