毫米 (háo mǐ) is the standard Chinese term for 'millimeter', a metric unit of length equal to one-thousandth of a meter. The character 毫 means 'tiny' or 'minute' and historically denotes a very small fraction (1/1000), while 米 means 'meter' — the base unit of length in the metric system. Together, they literally convey 'a tiny meter', reflecting the decimal relationship inherent in the metric system.

This unit is widely used in technical, scientific, engineering, and everyday contexts — such as measuring thickness of paper, precision of machine parts, rainfall depth, or screen pixel density. Unlike traditional Chinese length units (e.g., 寸 cùn), 毫米 is exclusively part of the modern metric system and is always written in this two-character form without variation. It’s commonly abbreviated as 'mm' in technical writing but never shortened in spoken Chinese.