1.the time that a play or film will begin in a theatre or cinema 〔戏剧或电影的〕开演时间,开场时间
2.American EnglishAmE informal the time when an activity should begin 〔活动的〕开始时间
Examples from the Corpus
showtime• The Boston show was canceled 10 minutes before showtime because Kelly was ill.• Student rushtickets are available 15 minutes before showtime.• It's five minutes to showtime and a hugeroar goes up as Ice-T announces the acts.