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. Millimeters are often used in various fields, including engineering, manufacturing, and medicine. For example, the size of surgical instruments or the dimensions of mechanical parts may be specified in millimeters to ensure precision and accuracy in design and function.