A generic term for any type of energy produced in conditions that the known laws of physics dictate should not be able to produce energy. Energy efficiency greater than 100% in any system is theoretically impossible because an energy source can only produce as much energy as it contains. When total energy produced exceeds the total energy that a given system ought to produce based on known principles, the excess energy is called overunity energy.

Inventors and hoaxsters have claimed to produce overunity energy almost humanity first discovered fire, and every year several reports of successful overunity generation appear in the press. No claims of success have ever been proven, but some theorists believe that what appears to be overunity energy may be actual energy produced from some as-yet unknown source of natural energy.

The energy industry's official position is that overunity energy is impossible to produce, but continuing advances in our understanding of physics could result in new, explainable discoveries of natural phenomena that would appear to us today to be instances of overunity generation. One example could turn out to be a variation on cold fusion energy. Many respected scientists still believe that cold fusion is a viable example of overunity energy that will eventually be explained by new discoveries in physics.

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