Perpetual motion describes "motion that continues indefinitely without any external source of energy; impossible in practice because of friction.It can also be described as "the motion of a hypothetical machine which, once activated, would run forever unless subject to an external force or to wear". There is a scientific consensus that perpetual motion is impossible, as it would violate the first or second law of thermodynamics.

Cases of apparent perpetual motion can exist in nature, but either are not truly perpetual or cannot be used to do work without changing the nature of the motion. For example, the motion of a planet around its star may appear perpetual, but not only is there friction due to particles in space, more fundamentally gravitational radiation will eventually cause all orbits to decay, in an analogous way to a rotating charge emitting electromagnetic radiation. This process has been observed in the most extreme cases, but planets orbiting the Sun only emit about 5000 W, making the decay of planetary orbits by this mechanism very slow. The fly-by of a space probe past a planet can be used to speed up the probe but in doing so alters the motion and reduces the energy of the planet in its orbit around the Sun. The flow of current in a superconducting loop can be used as an energy storage medium, but just as with a battery, using it to power a device will remove the equivalent amount of energy from the current in the loop.

Machines which extract energy from seemingly perpetual sources—such as ocean currents—are capable of moving "perpetually" (for as long as that energy source itself endures), but they are not considered to be perpetual motion machines because they are consuming energy from an external source and are not isolated systems (in reality, no system can ever be a fully isolated system). Similarly, machines which comply with both laws of thermodynamics but access energy from obscure sources are sometimes referred to as perpetual motion machines, although they also do not meet the standard criteria for the name.

Despite the fact that successful isolated system perpetual motion devices are physically impossible in terms of the current understanding of the laws of physics, the pursuit of perpetual motion remains popular. Assessment of technical designs of perpetual motion machines, in which the complete set of function of state transitions and their constrains byequivalence relation are examined, can lead to definite conclusions about its functionality without the need for additional physical examinations or test runs of actually build devices.