mm

mm — noun

1. written abbreviation for millimetre — a unit for measuring short lengths or smal

1.nounA2
Definition

written abbreviation for millimetre — a unit for measuring short lengths or small distances, equal to one thousandth of a metre.

Example

The rainfall measured only 2 mm overnight, so the garden is still dry.

number + mm measuring natural quantity (rainfall)

Selim cut a 15 mm strip of wood for the shelf he was building.

number + mm as attributive adjective before noun

Common collocations
  • per mm
  • ... mm long / wide / thick / deep
  • ... mm in diameter
  • ... mm strip of ...
  • ... mm of rain
Synonyms
  • millimetre

    the full written form; 'mm' is the standard abbreviation used in measurements

Grammar Patterns

number + mm + (long/wide/thick/deep)

number + mm

Usage Note

Used identically in British English (millimetre) and American English (millimeter). The space between the number and 'mm' is optional in informal writing but recommended in formal and technical contexts.

Common Mistakes

The board is 5 M thick.
The board is 5 mm thick.
A capital M means 'metres', not 'millimetres'. The abbreviation 'mm' uses two lowercase letters.
It measures 0.5 cm' when you mean 5 mm.
It measures 5 mm.
Note that 0.5 cm equals 5 mm; using mm is clearer for small lengths.

2. mm is short for millimolar — a unit used mainly in chemistry to show how concent

2.nounC1
Definition

mm is short for millimolar — a unit used mainly in chemistry to show how concentrated a solution is. One millimolar equals one thousandth of a mole of a substance in each litre of liquid.

Example

Quinn prepared a 5 mm solution of sodium chloride for the experiment.

number + mm + solution + of + substance (concentration pattern)

The lab report stated that the calcium level was 2.5 mm.

number + mm used alone, referring to concentration

Common collocations
  • mm solution
  • final concentration of ... mm
  • adjusted to ... mm
  • ... mm glucose / sodium chloride / buffer

Grammar Patterns

number + mm + of + substance

number + mm

Usage Note

Almost exclusively used in written lab work and scientific publications. In spoken chemistry, the full word 'millimolar' is preferred.

Common Mistakes

The solution was 5 M' (when you mean 5 mm).
The solution was 5 mm (millimolar).
A capital M means 'molar' (moles per litre), while a lowercase 'mm' means 'millimolar' (one thousandth of that).

Word family

  • millimetre
    nounB1
  • millimeter
    nounB1
  • millimolar
    adjectiveC1

Synonym discussion

Sense: noun/1

While 'mm' (millimetre) is an abbreviation for one specific unit of length, learners often need to compare it with other length units. A millimetre (mm) is the smallest common metric unit, about the thickness of a credit card. A centimetre (cm) equals 10 mm — roughly the width of a fingernail. A metre (m) equals 1,000 mm — about the height of a door handle. A kilometre (km) equals 1,000,000 mm — used for road distances. An inch (2.54 cm ≈ 25.4 mm) is the imperial equivalent. For everyday small measurements (screw sizes, rainfall, paper thickness), 'mm' is preferred in Taiwan; for body measurements and small objects, 'cm' is more common.

cm ↗mkm ↗inch ↗

Common collocations

Nouns

Phrases

  • ... mm long
  • ... mm wide
  • ... mm thick
  • ... mm deep
  • ... mm in diameter
  • ... mm of rain
  • ... mm strip

Prepositions

  • per mm
  • in mm
  • to the nearest mm

Etymology

LatinFrenchEnglish

The abbreviation 'mm' combines the SI prefix 'milli-' (from Latin 'mille' meaning 'thousand') with the first letter of the base unit. For the length sense, 'm' stands for 'metre' (from Greek 'metron', meaning 'measure'). For the concentration sense, 'm' stands for 'molar' (from Latin 'moles', meaning 'mass'). Both senses were standardised in the 19th century as the metric system spread internationally.