The phrase "rotor diameter" refers to the size of a spinning part in a machine, which is often circular in shape. It tells us how large the rotating part is from one side to the other, like measuring the width of a spinning wheel. Full definition
Their massive turbines, rated at 3.6 MW and with rotor diameters of 120 -LSB-...] (inspiredeconomist.com)
Wind - generated electricity is also dropping in price due to «turbines that have scaled, especially in rotor diameter and to a lesser extent in hub height.» (greentechmedia.com)
Front, 13.0 X 1.2 Brake Rotor Diameter X Thi (cars.trovit.com)