Wiktionary
n. A hypothetical device that exhibits perpetual motion, especially one that does useful work with no input of energy.
WordNet
n. a machine that can continue to do work indefinitely without drawing energy from some external source; impossible under the law of conservation of energy
Usage examples of "perpetual motion machine".
For what was the heart but the model for the perpetual motion machine?
Why can't we, if we're very clever, build a perpetual motion machine `of the first kind' (one that generates more energy than is supplied to it), or a perpetual motion machine `of the second kind' (one that never runs down)?
Nor did I mean to imply that there was a perpetual motion machine in the grotto.
Merely that something in the cave is capable of evolving power in an undiscovered fashion, which might ultimately lead to the development of a perpetual motion machine.
Thereafter, anyone who suggested such a thing was simply regarded as being uneducated, and was treated the way that humans would treat someone who wanted to build a perpetual motion machine.