A pulley is a simple machine.
The definition of a simple machine is a basic mechanical device.
An example of a simple machine is a pulley.
any of the basic mechanical devices, including the lever, wheel and axle, pulley, wedge, screw, and inclined plane, one or more of which are essential to any more complex machine
Any of numerous basic devices that alter the magnitude or direction, or both, of an applied force, traditionally including the lever, wedge, inclined plane, wheel and axle, pulley, and screw.