- 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.
- motion of a hypothetical mechanism that continues indefinitely without any external source of energy. It is impossible in practice because of friction
- motion of a hypothetical mechanism that derives its energy from a source at a lower temperature. It is impossible in practice because of the second law of thermodynamics
