A millimeter is a unit of measurement in the metric system, equal to one-thousandth of a meter. It is commonly used to measure small lengths, such as the thickness of a credit card or the diameter of a pencil lead. For example, a typical pencil lead is about 0.5 millimeters in diameter, making millimeters a practical choice for precise measurements in everyday life. In various fields like engineering and science, millimeters are essential for accuracy. For instance, when designing machines or electronic devices, engineers often use millimeters to ensure that parts fit together perfectly. This level of detail helps create reliable and efficient products.