Formal:
* Preamble: This emphasizes the introductory nature of the text.
* Preface: This is often used for written works.
* Introduction: This is a general term for the beginning of something.
* Overture: This is specifically used for musical works.
* Prelude: This is similar to overture but can also refer to a literary work.
Informal:
* Opening: This is a simple and straightforward term.
* Beginning: This is a general term for the start of something.
* Foreword: Similar to preface, but often written by someone other than the author.
Other Options:
* Setup: This emphasizes the role of the prologue in establishing the story.
* Exposition: This focuses on the introduction of characters and setting.
* Precursor: This emphasizes the prologue as a lead-in to the main story.
The best word to use will depend on the specific context and what you want to emphasize.