Editing Diameter (section)

{{Short description|Straight line segment that passes through the centre of a circle}}
{{Other uses}}
{{use British English|date=March 2021}}<!-- so centre not center -->
[[File:Circle-withsegments.svg|thumb|right|Circle with {{legend-line|black solid 3px|[[circumference]] ''C''}}
{{legend-line|blue solid 2px|diameter ''D''}}
{{legend-line|red solid 2px|radius ''R''}}
{{legend-line|green solid 2px|centre or origin ''O''}}]]
{{General geometry}}
In [[geometry]], a '''diameter''' of a [[circle]] is any straight [[line segment]] that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest [[Chord (geometry)|chord]] of the circle. Both definitions are also valid for the diameter of a [[sphere]].

In more modern usage, the length <math>d</math> of a diameter is also called the diameter. In this sense one speaks of {{em|the}} diameter rather than {{em|a}} diameter (which refers to the line segment itself), because all diameters of a circle or sphere have the same length, this being twice the [[radius]] <math>r.</math>

:<math>d = 2r \qquad\text{or equivalently}\qquad r = \frac{d}{2}.</math>

The word "diameter" is derived from {{langx|grc|διάμετρος}} ({{Transliteration|grc|diametros}}), "diameter of a circle", from {{lang|grc|διά}} ({{Transliteration|grc|dia}}), "across, through" and {{lang|grc|μέτρον}} ({{Transliteration|grc|metron}}), "measure".<ref>{{cite web |title=Diameter—Origin and meaning of diameter by Online Etymology Dictionary |url=http://www.etymonline.com/index.php?term=diameter |website=www.etymonline.com}}</ref> It is often abbreviated <math>\text{DIA}, \text{dia}, d,</math> or <math>\varnothing.</math>