WordNet (r) 3.0 (2006):

showstopper
    n 1: an act so striking or impressive that the show must be
         delayed until the audience quiets down [syn: show-
         stopper, showstopper, 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, showstopper]

The Jargon File (version 4.4.7, 29 Dec 2003):

showstopper
 n.

    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.


The Free On-line Dictionary of Computing (30 December 2018):

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)