An over-unity reaction is a reaction that produces strictly more output signals than input signals. By implication, "over-unity" refers to the ratio of outputs to inputs. This is an important concept in gun and macro-spaceship construction.
Such a reaction involves some input signals (e.g. gliders) as well as an optional sustained reaction (usually a catalyst, sparker, hassled object, or reburnable fuse). An over-unity reaction can be made self-sustaining by connecting 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 the output stream(s). If all signal outputs must be used up to sustain a stationary reaction, it can lead to an emu instead, for instance the Simkin's p60.
In a macro-spaceship, the synchronized reactions between two or more active fuses produce additional output signals, for example the 31c/240 Herschel-pair climber. Here "over-unity" implies that cleaning up the reburnable reactions does not use up all of the outputs; there are leftovers to build the fuse-supporting track.