an introduction to a play
"The director decided to keep the prologue as it was, since the actor's powerful delivery instantly set the tone for the entire play."
A speech or section used as an introduction, especially to a play or novel.
"Before the main story began, the author read a brief prologue that set the dark tone for the entire novel."
To introduce with a formal preface, or prologue.
"The editor decided to include a brief prologue that introduces the memoir's themes before the main narrative begins."