A Figure of Merit (FoM) is a calculated parameter used to characterize some aspect of optimization or (less often) aggregation with respect to the dingus. This notion is distinct from direct characterization of the dingus itself. FoM can be hideously complicated, which is usually a good sign that things might have become overly confuscated.
See, for example, the distinction between Measures of Performance and Measures of Efficiency, in which the latter are simple FoM.