Source : WordNet®

critical mass
     n 1: the minimum amount (of something) required to start or
          maintain a venture; "the battle for the computer market
          has now reached critical mass"; "there is now a critical
          mass of successful women to take the lead"; "they sold
          the business because it lacked critical mass"
     2: the minimum mass of fissionable material that can sustain a
        chain reaction

Source : Free On-Line Dictionary of Computing

critical mass
     
        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)