What does diameter mean?
A diameter is the straight line that crosses a circle or sphere from one side to the other while passing through its centre; by extension, the word also refers to the length of that line, making it a standard way of stating how wide something round is. In mathematics it holds a special place: the ratio of a circle's circumference to its diameter is the constant pi, roughly 3.14159, a relationship known since antiquity. Beyond geometry, diameter appears constantly in practical contexts — engineers specify pipe diameters, astronomers quote planetary diameters, and opticians measure lens diameters. It usually takes the construction 'a diameter of X centimetres' when paired with a measurement. The word derives from Greek dia- ('across') and metron ('measure'), literally meaning a measurement taken across. Precise, technical, and universally understood, it is indispensable wherever round objects are described.
nounA straight line passing from one side of a circle or sphere through the centre to the other side; also, the length of this line. More broadly, it denotes the distance across a circular or cylindrical object measured through its centre.
- A straight line passing through the centre of a circle or sphere and joining two points on its surface.
- The length of such a line — the measure of how wide a circular object is across its centre.
"The tree trunk had a diameter of nearly two metres."
"Cut the pipe so that each section keeps the same internal diameter."
"Jupiter has a diameter of roughly 140,000 kilometres."
Standard plural, used when comparing measurements of different objects.
"The two pipes have very different diameters."
Every circle's circumference is just over three times its diameter — that fixed ratio is pi, one of the most famous numbers in mathematics.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of diameter
Diameter comes from the Greek diametros, formed from dia- ('through' or 'across') and metron ('measure'), so it originally meant literally 'a measure across'. The word entered English via Latin in late Middle English, around the fifteenth century. It shares its Greek prefix with words such as diagonal and diagnosis, and its root metron with metre and metric.
Related word forms
How diameter is actually used
Diameter is a neutral, technical term common in mathematics, engineering, astronomy, and everyday measurement. It typically appears with 'of' plus a measurement ('a diameter of five centimetres') rather than as an adjective before a noun.
Easily confused with diameter
The radius runs from the centre to the edge of a circle and is exactly half the length of the diameter, which passes all the way across.
Circumference is the curved distance around the outside of a circle, whereas diameter is the straight distance across it through the centre.