Real Dictionary

perpetual motion
uncountable noun: The idea of perpetual motion is the idea of something continuing to move for ever without getting energy from anything else.

perpetual motion in American English
the motion of a hypothetical device which, once set in motion, would operate indefinitely by creating its own energy in excess of that dissipated
the motion of a theoretical mechanism that, without any losses due to friction or other forms of dissipation of energy, would continue to operate indefinitely at the same rate without any external energy being applied to it

perpetual motion in British English
noun: motion of a hypothetical mechanism that continues indefinitely without any external source of energy. It is impossible in practice because of friction