millimeter
n. countablen. a very small unit of length. There are ten of these in one centimeter and one thousand in a meter.
n. a metric unit of length equal to one thousandth of a meter. Often used for precise measurements in technical or scientific contexts.
The insect is only one millimeter long.
The carpenter measured the gap carefully to ensure it was within a millimeter of the plan.
Modern manufacturing requires components to be machined with a tolerance of less than a millimeter to ensure the mechanical parts fit together perfectly.
From French millimètre, as if milli- + meter.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.
Commonly abbreviated as 'mm' in both technical and everyday writing.
ten millimeterten millimetersWhen used as a noun with a number greater than one, it must be plural; the singular form is only used when it acts as a modifier before another noun.
Last updated May 8, 2026