diameter (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 indiameter. Visual vocab page v58 compare radivs 2 (technical) a measurement of the power of an instrument to magnify something a lens magnifying 300 diameters (= making something look 500 times larger than it really is)