1. Never ceasing; eternal with respect to futurity. Mine is a love, which must perpetual be, If you can be so just as I am true. Dryden. 2. Continual; uninterrupted; perennial. Within those banks rivers now Stream, and perpetual draw their humid train. John Milton.

perpetual motion, the action of a device that, once set in motion, would continue in motion forever, with no additional energy required to maintain it. Such devices are impossible on grounds stated by the first and second laws of thermodynamics.