1 a a straight line passing from side to side through the centre of a body or figure, esp. a circle or sphere. b the length of this line.
2 a transverse measurement; width, thickness.
3 a unit of linear measurement of magnifying power (a lens magnifying 2000 diameters).
Derivatives:
diametral adj.
Etymology: ME f. OF diametre f. L diametrus f. Gk diametros (gramme) (line) measuring across f. metron measure
Useful english dictionary. 2012.