What does millimetre mean?
A millimetre is a metric unit of length equal to one thousandth of a metre. It is written in full as millimetre in British English and millimeter in American English, with the abbreviation mm used in both varieties. Millimetres are used in contexts where small measurements require precision, including medicine, manufacturing, meteorology, construction, and map scales. For example, a doctor may report that a growth is several millimetres across, while a weather service may record rainfall to the nearest millimetre. Although the unit is very small, one thousand millimetres make a metre. The term combines the prefix milli-, meaning one thousandth, with metre, the base unit of length in the metric system.
nounA unit of length equal to one thousandth of a metre, or 0.001 metre. The US spelling is millimeter.
- A metric unit of length equal to one thousandth of a metre.
"The snow fell to a depth of several millimetres overnight."
"The tablet is only a few millimetres thick."
"Rainfall totals were recorded to the nearest millimetre."
The standard US spelling is millimeters.
"The component must fit within a tolerance of three millimetres."
A millimetre is small, but stacking a thousand of them gives you exactly one metre.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of millimetre
The word was formed in the early 19th century from the Latin-derived prefix milli-, meaning one thousandth, and metre, which comes from Greek metron, meaning measure. The prefix milli- itself comes from Latin mille, meaning thousand, and is also the source of English words such as million. The metric prefix and the metre were adopted together as part of the international standardization of measurement.
How millimetre is actually used
Millimetre is the usual British spelling; millimeter is used in the United States. The unit is commonly abbreviated to mm, without a final period.
Easily confused with millimetre
Millimetre is the standard British spelling, while millimeter is the standard US spelling; both name the same unit of length.