Source : WordNet®

showstopper
     n 1: an act so striking or impressive that the show must be
          delayed until the audience quiets down [syn: {show-stopper},
           {stopper}]
     2: something that is strikingly attractive or has great popular
        appeal; "she has a show-stopper of a smile"; "the
        brilliant orange flowers against the green foliage were a
        showstopper" [syn: {show-stopper}]

Source : Free On-Line Dictionary of Computing

showstopper
     
        A hardware or (especially) software {bug} that makes an
        implementation effectively unusable; one that absolutely has
        to be fixed before development can go on.  Opposite in
        connotation from its original theatrical use, which refers to
        something stunningly *good*.
     
        [{Jargon File}]
     
        (1995-01-31)