A device that enables energy from one source to be modified and transmitted as energy in a different form or for a different purpose. Simple examples, in mechanics, are pulleys and levers, which may be used as components in a more complex machine. The force applied to the machine is the effort, and the force exerted by the machine is equal and opposite to the load.

If the effort has magnitude P and the load has magnitude W, the mechanical advantage of the machine is the ratio W/P. The velocity ratio is the ratio of the distance travelled by the effort to the distance travelled by the load. It is a constant for a particular machine, calculable from first principles. For example, in the case of a pulley system, the velocity ratio equals the number of ropes supporting the load or, equivalently, the number of pulleys in the system.

The efficiency is the ratio of the work obtained from the machine to the work put into the machine, often stated as a percentage. In the real world, the efficiency of a machine is strictly less than 1 because of factors such as friction. In an ideal machine for which the efficiency is 1, the mechanical advantage equals the velocity ratio.

See also Turing machine.