a rotary part of a machine or vehicle, in particular. a hub with a number of radiating airfoils that is rotated in an approximately horizontal plane to provide the lift for a rotary-wing aircraft. the rotating assembly in a turbine, especially a wind turbine. the armature of an electric motor. the rotating part of the distributor of an internal combustion engine that successively makes and breaks electrical contacts so that each spark plug fires in turn. the rotating container in a centrifuge. the rotary winder of a clockwork watch. Meteorology a large eddy in which the air circulates around a horizontal axis, especially in the lee of a mountain.