Real Dictionary

prologue
countable noun: A prologue is a speech or section of text that introduces a play or book.
countable noun: If one event is a prologue to another event, it leads to it.

prologue in American English
an introduction to a poem, play, etc.; esp., introductory lines spoken by a member of the cast before a dramatic performance
noun: a preliminary discourse; a preface or introductory part of a discourse, poem, or novel
transitive verb: to introduce with or as if with a prologue

prologue in British English
noun: the prefatory lines introducing a play or speech
verb: to introduce or preface with or as if with a prologue