A performance, act, or thing that is so impressive that it stops the show momentarily and deserves special attention or applause.
From 'show' plus 'stopper,' combining theater terminology with the sense of something that halts proceedings. It became popular in the early 20th century.
A literal showstopper in theater would make the audience applaud so hard the show had to pause—it's why the most dazzling performance number in a musical was always written to stop the show!
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