diameter

diameter — noun

1. the distance across a round shape, measured by a line passing through its centre

1.nounB1
Definition

the distance across a round shape, measured by a line passing through its centre and meeting the edge at two opposite points; also used for the numerical length of that line.

Example

Hari measured the diameter of the tree trunk before cutting it down.

measure the diameter of [round object]

A pipe with a wider diameter allows more water to flow through it.

wider diameter

Common collocations
  • measure the diameter
  • the diameter of
  • [number] cm/m in diameter
  • wide/narrow diameter
Synonyms
  • width

    general measurement from side to side; diameter specifically requires going through the centre of a round shape

  • breadth

    similar to width, but less precise; not restricted to round shapes

Grammar Patterns

the diameter of [something]

[number] in diameter

Common Mistakes

The diameter of the circle is half the radius.
The diameter of the circle is twice the radius.
diameter = 2 × radius, not the other way around.
Draw a diameter line across the square.
Draw a diagonal line across the square.
diameter only applies to round shapes, not squares or rectangles.

2. a unit that tells you how many times larger an object appears when seen through

2.nounC1
Definition

a unit that tells you how many times larger an object appears when seen through a lens, microscope, or telescope, compared to how it looks with the naked eye.

Example

Folake observed the cells under a microscope at four hundred diameters magnification.

[number] diameters magnification

The telescope's two hundred diameter setting made the craters on the moon easy to see.

[number] diameter setting

Common collocations
  • [number] diameters
  • at [number] diameters
  • lens of [number] diameters
Synonyms
  • magnification

    more common and general term; 'diameters' is restricted to lens specifications

  • power

    informal shorthand ('the scope's power is 20×'); less precise than 'diameters'

Grammar Patterns

[number] diameters

at [number] diameters magnification

Usage Note

This sense is almost always used with a number (e.g., '40 diameters', '100× diameter'). In modern technical writing, '×' or 'times' is often preferred over 'diameters'.

Common Mistakes

The microscope has a diameter of 400.
The microscope magnifies to 400 diameters.
the number refers to magnification power, not the physical size of the lens opening.

Word family

  • diametric
    adjectiveC1
  • diametrically
    adverbC1
  • diametrical
    adjectiveC1

Synonym discussion

Sense: noun/1

Diameter, width, radius, and circumference all describe measurements of size or distance, but they apply to different parts of a shape. Width is the most general term — it describes how wide something is from side to side, without needing a central point or a round shape. Radius is half of a diameter: the distance from the centre of a circle to its edge. Circumference is the distance all the way around the outside of a circle. Diameter specifically runs through the centre and connects two opposite points on the edge. For example, if a pizza has a diameter of 30 cm, its radius is 15 cm and its circumference is about 94 cm. Width would describe a rectangular table (90 cm wide), but not a circle as precisely.

diameter ↗width ↗radius ↗circumference

Common collocations

Nouns

Adverbs

Phrases

  • across the diameter
  • the diameter of [something]
  • [number] centimetres in diameter

Prepositions

  • in diameter
  • of

Etymology

GreekLatinFrenchEnglish

From Greek 'diametros' (dia- 'across' + metron 'measure'), meaning a line that measures across a circle. The word entered English through Latin 'diametrus' and Old French 'diametre' during the 14th century.