A millimeter 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 symbol for millimeter is "mm." Millimeters are often used in various fields, including engineering, manufacturing, and medicine. For example, in engineering, precise measurements in millimeters can be crucial for creating parts that fit together correctly. In medicine, millimeters may be used to measure the size of tumors or other anatomical features.