preface; foreword; introductory chapter
Refers to the opening chapter of a book or a formal introductory section. Often used in literary or academic contexts.
この本の序章を読めば、全体の構成がわかる。
If you read the preface of this book, you'll understand the overall structure.
Figurative use for the initial stage of something, such as a project, era, or story. Less common than the literal book sense.
これはまだ物語の序章にすぎない。
This is still just the beginning of the story.
I stated the purpose of the research in the introductory chapter of the thesis.
序章 refers to the introductory chapter of a book or story, whereas 序幕 specifically evokes the opening act of a stage play or a dramatic beginning.
Compound of 序 (jo, 'order' or 'beginning') and 章 (shō, 'chapter'). The kanji combination directly yields the meaning 'introductory chapter'.