A component of circular cross section that rotates and transmits power from a driving device, such as a motor or engine, through a machine.

A spindle is a rotating shaft with a fixture for holding a tool (in the case of a milling, grinding, or drilling spindle) or a work-piece (in the case of a turning spindle). The spindle shaft serves as a support, a positioner, and a rotary drive for the tool or work-piece.