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]