Refers to an orchestral opening piece, or metaphorically to an event that signals the beginning of something larger.
The orchestra began to play the opera's overture.
This small clash was merely a prelude to the great war that followed.
That small clash became the prelude to the war.
The hall went dark and the overture began.
The orchestra played the opera's overture.
前触れ is a general everyday word for any sign or harbinger of what's to come, whereas 序曲 is more formal and literary, often used metaphorically for the start of a significant event.
終曲 is the closing movement or finale of a musical work, the direct opposite of 序曲, which is the opening piece.
前奏 is a short musical introduction to a single song, while 序曲 is a full-scale overture introducing a larger opera or orchestral work.
Compound of 序 (jo, 'beginning' or 'order') and 曲 (kyoku, 'musical piece'). The word is a direct adaptation of the Western musical concept of an overture.