(in the past) a song or tune played or sung at night by a man outside the window of the woman he loves （旧时男子在所爱慕的女子窗下演奏或歌唱的）小夜曲 a gentle piece of music in several parts, usually for a small group of instruments （尤指供小型乐队演奏的）小夜曲
Word Originmid 17th cent.: from French sérénade, from Italian serenata, from sereno ‘serene’.