prologue

Concise Oxford English Dictionary © 2008 Oxford University Press:
prologue/ˈprəʊlɒɡ/

▶noun

• 1 a separate introductory section of a literary or musical work.

  ■ an introductory scene in a play.

• 2 an event or action leading to another.

– origin ME: from OFr., via L. from Gk prologos, from pro- ‘before’ + logos ‘saying’.