punch-line

punch-line
n words that form the climax of a joke or story 笑话或故事中形成高潮的语句; 画龙点睛的妙语
    He forgot the punch-line of his after-dinner speech. 他在宴会之後的讲话中把那句最绝妙的话给忘了.

牛津英汉双解词典