µm

IPA/mˌaɪkɹəʊˈɛm/
IPA/mˌaɪkɹoʊˈɛm/

µm — noun

1. a written symbol that stands for the unit of length called a micrometre (or micr

1.nounB2
Definition

a written symbol that stands for the unit of length called a micrometre (or micron); one µm equals one millionth of a metre, and scientists use it to describe objects that are too small to see without a microscope — such as bacteria, cells, and particles of dust or pollution.

Example

The lab report showed the dust particle was about ten µm wide.

µm + adjective of dimension (wide)

Doctors sometimes use µm to describe the size of human cells and tiny blood vessels.

use µm to describe [measurement]

Common collocations
  • measured in µm
  • µm wide / long / thick / across
  • less than one µm
  • a few µm
Synonyms
  • micrometre

    the full name that the symbol µm stands for; used in formal scientific writing

  • micron

    the older, shorter name for the same unit; still common in industry and some technical fields

Grammar Patterns

numeral + µm

µm + adjective of dimension

Usage Note

The symbol µ is the Greek letter mu. In science writing, µm always appears with a numeral; do not use it alone (write '50 µm', not 'several µm'). The alternative name 'micron' means the same thing and is sometimes used in informal technical contexts.

Common Mistakes

The particle measures 5 µm of length.
The particle is 5 µm long.
In English, units of measurement are placed before the adjective of dimension, not after 'of'.
The µm is a tool for measuring cells.
The µm is a unit for measuring cells.
µm is a unit (a quantity), not a tool or device.

Word family

Synonym discussion

Sense: noun/1

Micrometre (symbol µm) and micron are exact synonyms — they name the same unit (one millionth of a metre). 'Micrometre' is the formal SI name, while 'micron' is older and still common in manufacturing and biology. Nanometre (nm) is one thousand times smaller (one billionth of a metre) and is used for molecules and semiconductor circuits. Millimetre (mm) is one thousand times larger and used for everyday measurements like rainfall or paper thickness. To choose: if you are measuring cells, pollen, or dust, use µm (micrometre/micron). For bacteria and viruses, you may need nm. For visible distances like button size, use mm.

micrometremicron ↗nanometremillimetre ↗

Common collocations

Phrases

  • measured in µm
  • µm wide / long / thick / across
  • less than one µm
  • a few µm
  • µm in diameter

Etymology

GreekFrenchEnglish

The symbol µ comes from the Greek letter mu (μ), which international science uses as the prefix meaning 'one millionth'. The letter m stands for 'metre', the base SI unit of length. The full term 'micrometre' was coined in the late 1800s from Greek mikrós 'small' + 'metre'. The shorter name 'micron' was common through most of the 20th century but has gradually been replaced by 'micrometre' in formal science, though the symbol µm stays the same.