Formal:
* Prologue: This is the most common term for a short chapter that sets the scene before the main story begins.
* Preface: Similar to a prologue, but often focuses more on the author's intentions or the book's background.
* Foreword: Written by someone other than the author, often a respected figure in the field.
* Introduction: Used for setting the context of the main body of the book.
Informal:
* Flashbacks: Used for short chapters that tell a story from the past.
* Expository Chapters: A more straightforward term for chapters that provide background information.
* Backstory Chapters: Focuses on the history of the characters or setting.
Ultimately, the best term to use depends on the specific content of the chapter and your preferred style.