A wheel with a broad rim, or grooved rim, for transmitting power from, or imparting power to, the different parts of machinery, or for changing the direction of motion, by means of a belt, cord, rope, or chain.
a wheel over which a rope, chain or belt passes. Pulleys can be used to change the speed of a mechanism. If a large pulley is connected by a belt to a small pulley, the small pulley turns faster causing an increase in speed. If a small pulley drives a large one, there is a decrease in speed but an increase in power.