milimetre

The millimetre, abbreviated mm and spelled millimetre in British English, is a unit of length in the metric system. It is defined as one thousandth of a metre; thus 1 mm = 0.001 m and 1 m = 1000 mm. The prefix milli- denotes a factor of 10^-3.

In the International System of Units (SI), the millimetre is a derived unit, not a base unit. It is convenient for measuring small distances on human scales. Related units include the centimetre (1 cm = 10 mm) and the micrometre (1 µm = 0.001 mm). The inch is exactly 25.4 mm.

Usage: Millimetres are widely used in engineering, manufacturing, electronics, construction, and science to specify dimensions, thickness, tolerances, and gaps. Measuring tools such as calipers and micrometers often display values in millimetres or fractions of a millimetre (for example, 0.1 mm, 0.05 mm). For very fine features, measurements may be expressed in micrometres or nanometres.

Examples and scope: A standard credit card is about 0.76 mm thick; typical paper is about 0.1 mm thick; a human hair generally ranges from about 0.017 to 0.18 mm in diameter. The millimetre provides a practical reference for both everyday and technical measurements.

Spelling and usage: "Millimetre" is the preferred form in British English; "millimeter" in American English. In formal contexts the symbol "mm" is used for both singular and plural, in common with other SI units.