One way to hold showtime: the house lights go down, the talking stops, and the curtain moves. Up to that second everything was preparation — tickets, seats, tuning, waiting. After that second, people are watching and the thing itself is happening. Showtime is that dividing second, first on a real theatre clock, and then anywhere at all.
showtime 的核心畫面可以想成：場燈暗下來，說話聲停了，布幕開始動。在那一秒之前，全部都是準備——買票、入座、調音、等待。在那一秒之後，有人在看，事情真的開始了。showtime 就是那一秒的分界線。它先是戲院時鐘上的一個刻度，後來被搬到任何地方去用。
From the word’s history
The source records showtime as simply show + time. The two parts say the whole thing: the time at which the show begins.
來源記載 showtime 就是 show 加 time。兩個部分合起來就說完了：表演開始的那個時間。這也是為什麼它後來可以被借去講任何「該開始了」的時刻——只要把「表演」換成你正要做的那件事，畫面完全不用改。
Wiktionary, "showtime"
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