a straight line going from one side of a circle or any other round object to the other side, passing through the centre 直径；对径 
The diameter of the tree trunk was more than a metre. 树干的直径超过一米。 - in diameter
The dome is 42.3 metres in diameter. 这个穹顶直径为 42.3 米。
Extra ExamplesTopics Colours and Shapesc1, Maths and measurementc1The mirror is 25cm in diameter. 镜子的直径是25 厘米。 The tubes have an internal diameter of 2 mm. 管子的内径是 2 毫米。 a method of calculating the diameter of the earth 计算地球直径的一种方法 The radius of a circle is half its diameter. 圆的半径是直径的一半。
Oxford Collocations Dictionaryadjective- inner
- inside
- internal
- …
- calculate
- determine
- estimate
- …
- in diameter
- (specialist)
a measurement of the power of an instrument to magnify something 放大率；放大倍数 a lens magnifying 300 diameters (= making something look 300 times larger than it really is) 放大 300 倍的透镜
Word Originlate Middle English: from Old French diametre, via Latin from Greek diametros (grammē) ‘(line) measuring across’, from dia ‘across’ + metron ‘measure’.