An over-unity reaction is a stationary or moving reaction between some number of input signals (e.g. gliders) as well as an optional sustained reaction (usually a catalyst, sparker, hassled object, or reburnable fuse reaction) that produces a strictly greater number of output signals than input signals. This is an important concept in gun and macro-spaceship construction. By implication, "over-unity" refers to the ratio of outputs to inputs. An over-unity reaction can be made self-sustaining by manipulating signals to connect outputs to inputs.
In the stationary case, mechanisms (reflectors, conduits, etc.) can usually be added to produce a gun, where the excess signal(s) become a gun's output stream. If all signal outputs must be used up to sustain a stationary reaction, a high-period emu oscillator may still be possible. See Simkin's p60 for a small example.
In a macro-spaceship, the outputs may be used in a variety of ways to construct the spaceship's supporting track. In this case it's necessary but not sufficient for a traveling reaction to restore a reburnable fuse to its initial state. There also has to be some way to produce additional output signals, usually via reactions synchronized between two or more active fuses. See 31c/240 Herschel-pair climber for an over-unity reaction that produces pairs of backward gliders as two block trails are repeatably burned. In a macro-spaceship context, "over-unity" means that all outputs of a fuse can't be used in cleaning up a reburnable reaction; there have to be some outputs left over to use to build the front end of the fuse.