Definition of diameter in US English:


(also diam.)


  • 1A straight line passing from side to side through the center of a body or figure, especially a circle or sphere.

    • ‘The sine wave through the diameter of the circle is the ideal and basic pulse wave.’
    • ‘Therefore, all diameters of a centrally symmetric shape of constant width pass through the center of symmetry.’
    • ‘He was the first to resolve Kepler's Problem on cutting a semicircle in a given ratio by a line through a given point on its diameter.’
    • ‘Users can measure the distance of vertices/edges/faces, the angle of edges/faces, and the radii and diameters of circles.’
    • ‘They were represented by red circles having a diameter of 5 mm presented on a black screen.’
    • ‘The diameter of a sphere is 6.25 nm, and panicles cannot be closer than that.’
    • ‘Given a circle, find a point outside the circle where the tangent to the circle and diameter produced, have a given ratio.’
    • ‘In order to accomplish this end, the diestro must move off the diameter of the circle and place himself at an angle to his adversary.’
    • ‘The rule is to cut 1/9 off the circle's diameter and to construct a square on the remainder.’
    • ‘In book one the relations satisfied by the diameters and tangents of conics are studied while in book two Apollonius investigates how hyperbolas are related to their asymptotes, and he also studies how to draw tangents to given conics.’
    • ‘Also, the ortholine serves as the common radical axis of the three circles constructed on the diagonals as diameters, such that whenever the circles intersect, all three of them intersect in two points on the ortholine.’
    • ‘The circle with diameter BC intersects the sides AB and AC at M and N respectively.’
    • ‘He wrote further articles on cubic curves and in this area he wrote the memoir On the diameters of cubic curves which was published in the Transactions of the Royal Society in 1889.’
    • ‘We owe to him a note on the curvature of elastic rods, several works on the flow of air, and finally, in 1848, an important posthumous note on the rectilinear diameters of curves.’
    • ‘In 30 feet of water the circle has a diameter of 3 feet.’
    • ‘The dimensions also suggest that the intact liposome as a sphere should have a diameter of several hundred nanometers.’
    • ‘The distance from the ground to the tip of his extended index finger is the diameter of the circle.’
    • ‘And this he proved by first showing that the squares on the diameters have the same ratio as the circles.’
    • ‘One day at school we were told that if AB is a diameter of a circle, and C is any point on the circumference, then the angle ACB is a right angle.’
    • ‘Can we draw a line thinner than the diameter of a hydrogen atom?’
    1. 1.1 The length of the diameter of a figure, especially a circle or sphere.
      • ‘At its simplest, Pi is the ratio of the circumference of a circle to its diameter.’
      • ‘No one expected the value p, the ratio of the circumference of a circle to the diameter, to appear in the formula for the sum.’
      • ‘The main results of book twelve are that circles are to one another as the squares of their diameters and that spheres are to each other as the cubes of their diameters.’
      • ‘Measure its diameter (the width across the center of circle) with the ruler.’
      • ‘Therefore the precise value of the ratio of the circumference of a circle to its diameter is as 355 to 113, and the approximate value is as 22 to 7.’
      • ‘There were twenty-four, 4-inch diameter pipes that were completely buried beneath the sand.’
      • ‘Their diameters varied between 4.4 and 19.1 m, with a mean of 13.050.29 m.’
      • ‘Horrocks purchased a simple telescope which he set up to project an image of the sun onto a graduated circle six inches in diameter.’
      • ‘If we cut the circles along the radius of the disk, and let them fan out to become straight lines, we get a triangle (because the ratio of the circumferences of the circles to their diameters is constant).’
      • ‘The ratio of the diameter of the larger circle to the smaller is 3: 2.’
      • ‘Since he did not stipulate how the diameters of the circles are to be determined, Newton's theory of colour-mixing has no empirical significance.’
      • ‘The diameter of the inscribing sphere of the rhombic dodecahedron was 8.34 nm.’
      • ‘Learning about things like the Civil War and how to calculate the diameter of a circle just wasn't of interest to me.’
      • ‘A translucent circle with the diameter of approximately five feet with lines rippling across it appeared from out of the cylinder.’
      • ‘In the normal geometry of flat space, the diameter of a circle is its circumference divided by pi.’
      • ‘Root area was calculated by using the diameter of a circle with equal circumference.’
      • ‘The first figure is the diameter of the cylinder, and the second the stroke or movement of the piston from one end of the cylinder to the other.’
      • ‘In the diagram d is the diameter of the circle, a is the length of the arc AB and x is the length of NB which Guo wanted to calculate.’
      • ‘The diameter of the circle, an exact multiple of a particular wavelength, determines the wavelength of light permitted to pass.’
    2. 1.2 A transverse measurement of something; width or thickness.
      • ‘Woody stem diameters were measured with calipers to the nearest millimeter immediately above the root collar.’
      • ‘We estimated average tree diameter for canopy and subcanopy trees combined.’
      • ‘In contrast, algal cells tend to be similar in size, so the diameter of a clump of algal cells would depend on the number of cells in the cluster.’
      • ‘Objects from 1 to 10 cm in size - about the diameter of a salad bowl - cause the real worry.’
      • ‘Furthermore, none of the particles may have a diameter greater than the thickness of the uncured fluid layer.’
      • ‘They are two to three times the diameter of Earth - the size of Neptune.’
      • ‘Prune out all canes with diameters less than a pencil width and old, brown canes that tend to be less productive.’
      • ‘The first dorsal fin has four rays, the lips are smooth and are roughly the same thickness as the diameter of the eye.’
      • ‘Flowers are erect, and diameters range from 1.9 to 2.7 cm when tepals are completely expanded.’
      • ‘Bark thickness and stem diameter were recorded each time a probe was installed or removed.’
      • ‘The correct length of a buttonhole is determined by the diameter, thickness and type of button used.’
      • ‘The fluted barrel would have a larger outside diameter, and thus be stiffer.’
      • ‘Linzon carefully observed the condition of the bark and other parts of the trees, measured their diameters and took bore samples to measure tree rings.’
      • ‘The best of these fruits, which range in size from the diameter of a pea to that of a cherry tomato, can compete with blooms for beauty.’
      • ‘The rosette diameter was measured after full leaf expansion using a ruler with 1 mm precision.’
      • ‘Table saw size is specified by the maximum diameter of the blade they will accept.’
      • ‘Microscopic examination revealed that the increase of the root diameter was due to an increased thickness of the cortex.’
      • ‘"Transco has 280 kilometres of 48 inch diameter pipe in the UK and we have had no problems.’
      • ‘Work the screwdriver around to increase the diameter of the hole until it is slightly smaller than the wooden dowel.’
      • ‘Estimated leaf mass of first-order branches is plotted against basal stem diameter of the branch in Fig.10.’
      breadth, width, depth, thickness
      View synonyms
  • 2A unit of linear measurement of magnifying power.

    • ‘In the photograph above, for instance, eggs from the Central American stick-insect genus Bacteria are shown, magnified roughly fifteen diameters; the brown, knobby protruberances are the capitula.’
    • ‘The smaller diameter objective lens is less likely to reflect light.’
    • ‘A large aperture eyepiece will increase the filed of view and a large diameter, well coated objective lens will enhance brightness issue.’
    • ‘Using a micrometer to measure the field diameter of the microscope is recommended.’
    • ‘Customized spacers can be used to fill measured gaps, or the mount diameters can be machined at assembly to closely match the measured diameters of the lenses.’


Late Middle English: from Old French diametre, via Latin from Greek diametros (grammē) ‘(line) measuring across’, from dia ‘across’ + metron ‘measure’.