What does millimeter mean?
A millimeter is a metric unit of length equal to one thousandth of a metre. It is used to describe small dimensions, thicknesses, distances, and tolerances, especially in science, engineering, medicine, and manufacturing. The abbreviation mm is common in technical writing and on measuring instruments, as in “10 mm,” while the full word is preferred in prose and labels that need to be read easily. The unit is part of the metric system, so it combines naturally with other metric measurements such as centimeters, meters, and kilometers. Although a millimeter is physically small, it can be important in contexts requiring precision: a tiny difference in a component’s dimensions may affect how a machine operates or how a medical device fits. In everyday speech, millimeter is often contrasted with centimeter, with the former suggesting a much smaller measurement.
nounA metric unit of length equal to one thousandth of a metre, or 0.001 metre. It is often used for small measurements in engineering, science, medicine, and precision manufacturing.
- A metric unit of length equal to one thousandth of a metre.
"The blade was less than a millimeter thick, making it delicate to use."
"The hole was only two millimeters across."
"The manufacturer tested each component to within a fraction of a millimeter."
The standard US plural is millimeters; the British spelling is millimetres. In technical writing, the abbreviation “mm” is generally used for both singular and plural quantities.
"The machine can adjust the position within a few millimeters."
A millimeter is small, but precision measurements can make its difference.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does millimeter come from?
The word millimeter was formed in the early 19th century from the Latin mille, meaning “thousand,” and the Greek metron, meaning “measure.” It was modeled on related metric terms such as centimeter and kilometer, with the prefix-like element milli- indicating one thousandth. The spelling millimeter is standard in US English, while millimetre reflects British usage.
Related word forms
How millimeter is actually used
The symbol mm is used in scientific and technical contexts and is the same in British and US English. The spelling varies by region: millimeter in the US and millimetre in Britain and other countries following British usage.
Easily confused with millimeter
Millimeter is the usual US spelling, while millimetre is the standard British spelling of the same unit.