Millimetre/ˈmɪlimiːtər/ Definition 1 of 3Pro Tip 1/3Precision Matters Millimetre is often used when accuracy is crucial in measurements for technical fields. Ensure the lengths are correct to the nearest millimetre. Pro Tip 2/3Visualize Size Picture a millimetre as about the thickness of a credit card to grasp its small size. Cut the paper five millimetres thick, like a credit card's edge. Pro Tip 3/3Common Usage Millimetre is frequently used in everyday life, from school rulers to rainfall measurements. The ruler shows that your nail is ten millimetres long.