/ daɪˈæmɪtə(r); NAmE / noun
1.
a straight line going from one side of a circle or any other round object to the other side, passing through the centre :
the diameter of a tree trunk
The dome is 42.3 metres in diameter .
—picture at circle
—compare radius
2.
( technical ) a measurement of the power of an instrument to magnify sth :
a lens magnifying 300 diameters (= making sth look 300 times larger than it really is) .
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WORD ORIGIN
late Middle English : from Old French diametre , via Latin from Greek diametros (grammē) (line) measuring across, from dia across + metron measure.