Overunity is a relative new term, according to Google not heard of until late 2007. The current definition of overunity would be: The difference of incoming and outgoing energy from an apparatus. Where a positive amount would indicate more energy coming out then going in. Theoretically impossible, as they conflict with the current laws of physics. Wikipedia even goes as far as to directly link it to the perpetual motion page.
As always these kind of hard definitions can hinder research done into a subject. Before judging the definition by its contradiction to existing presumptions, one can try to find the common denominator. E.g. take the experiments that are done with active magnet motors, powered by a battery but generating enough electricity to power a normal house light bulb for hours. Scientific reactions range from literally ignoring evidence to refuting it is free energy. My purpose it to show why such ignorance is not only counter productive, but also strange concerning how the universe itself works.
Thinking out of the box(apparatus). There is only one true, real, and unarguably defined perpetuum mobile; the universe itself. But still elements within this large machine must move seemingly endless. (Perhaps after a long time it will find equilibrium and stop but this is not a practical problem.) So if the parts of the machine show the properties of the whole, then for all intent and purpose they are equal in usability. As one element in the machine uses another element as its energy source, it must seem endless, where in fact it is using the energy of the whole. It is therefor the connection or transference point that we seek to describe, even though it is not necessary to be able to use it.
Therefor; Overunity is the effect of indiscriminately gaining more energy from a source then was put into it by the user.