A machine that creates more energy than it consumes.
A Perpetuum Mobile (Latin for "forever moving") is an old dream of mankind: A machine that creates more energy than it receives from the outside. (A weaker version just keeps moving on and on, without creating new energy you could put in use.) Obviously, a Sub-Trope of Artistic License - Physics. To be precise, a Perpetuum Mobile contradicts the Second Law of Thermodynamics. Of course, this didn't stop some people from believing in a PM - they just insist that obviously, the Second Law has to be wrong. Subtrope of Applied Phlebotinum. A Mad Scientist may work on this.