prologue
noun uk/ˈprəʊ.lɒɡ/ us/ˈproʊ.lɑːɡ/
[ C ]（戏剧的）开场白，开场诗，引子
a part that comes at the beginning of a play, story, or long poem, often giving information about events that happened before the time when the play, story, or poem begins
Compare: epilogue
[ S ] literary（重大事件的）开端，序幕
a series of events that happen before the main event and are related to it
informal A series of internal struggles was the prologue to full-scale civil war. 一系列内部斗争是爆发全面内战的序幕。