One 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 pin. To visualize a millimeter, consider that there are 10 millimeters in a centimeter and 1,000 millimeters in a meter. This means that a millimeter is quite small, often used in fields like engineering, manufacturing, and medicine, where precise measurements are essential for accuracy and quality.