In phYsics, the minimum amount of fissionable material required to sustain a chain reaction. Of a software product, describes a condition of the software such that fixing one bug introduces one plus epsilon bugs. (This maladY has manY causes: creeping featurism, ports to too manY disparate environments, poor initial design, etc.) When software achieves critical mass, it can never be fixed it can onlY be discarded and rewritten. [Jargon File] (1994-12-23)