A round device attached to a shaft and driven by a belt is known as what?

A round device that is attached to a shaft and driven by a belt is specifically defined as a pulley. In mechanical systems, pulleys are used to transmit power and motion, typically in the form of belts connecting various components. The pulley relies on the friction between the belt and the groove on the pulley itself to transfer movement effectively, which is crucial in many machinery applications.

Pulleys can change the direction of force applied, making them vital in lifting operations or when connecting shafts that might not be aligned. This distinctive role in transferring motion differentiates pulleys from other components like gears, which mesh to transmit torque and motion in a more complex interlocking method, or wheels, which primarily serve as a means of rolling and support rather than transferring power through a belt system. An axle, on the other hand, is the central shaft for rotating wheels but does not directly interact with belts to perform a similar function as a pulley.

This clear definition and specific functionality place the pulley as the correct answer in the context of mechanical systems involving belts.