The working definition
A chapter break is a boundary placed in the continuous run of a novel’s scenes and sequels, whose position, not its length and not the technique used to cross it, determines what a reader carries with her if she stops there. Placed means the break is a decision distinct from the shape of the material: scenes end where their change completes, chapters where the writer decides. In the continuous run means nothing about the events of a novel determines where its chapters fall. Not its length means extent is a separate variable, treated in The Mechanism. And what a reader carries if she stops is the operative clause: the break is an offered exit, and the last line sets its price.
The term most often confused: the scene
Most misdiagnosed chapter problems begin with a confusion between the chapter and the scene. A scene is a unit of story, bounded by its own logic: a character wants something, meets resistance, and the situation changes, so that the writer discovers where the scene ends by discovering where the change completes (The Purposes of a Scene, Chapter 1). A chapter is a unit of the book, imposed on the story from outside, and its boundaries are chosen. The two therefore come apart freely. A chapter may hold one scene, the common arrangement in contemporary commercial fiction; several scenes and their sequels, the nineteenth-century norm in Middlemarch or Vanity Fair; part of one scene, as at many of Dumas’s breaks; or no scene at all, as in the chapters of Moby-Dick that classify whales. One scene may even run across a break: in Don Quixote, Part I, chapters 8 and 9, a duel is cut in half by a chapter boundary and a change of manuscript. The practical consequence is that does this chapter work? is two questions. Does the material inside it work as story, which The Purposes of a Scene (Chapter 1) and Scene and Sequel (Chapter 2) ask, and is the boundary in the right place, which this chapter asks? A draft can pass the first and fail the second, and no revision of the scenes will fix it.
A second confusion is between the chapter break and the section break, the white space or ornament inside a chapter. The two marks make different offers. A section break says that the story has jumped and the chapter is still running; a chapter break says that a unit has ended and the reader may leave. A writer who uses chapter breaks where section breaks are meant keeps inviting the reader out of the book, and a writer who uses section breaks where chapter breaks are meant refuses to let her out at all, which is a quieter fault and as real.
Scales of division
Four scales are in common use: a paragraph break asserts continuity, a section break a gap inside a continuing unit, a chapter that a unit has ended, and a Part or Book that the book itself has changed, in phase, place, method or period. There is no principled length for a chapter, and every figure quoted in craft writing is a convention of a period and a market. Tristram Shandy has chapters of a single sentence; One Hundred Years of Solitude has twenty chapters of something like seven thousand words each; The Da Vinci Code runs to more than a hundred chapters averaging about thirteen hundred. These books are doing different things, not one thing with different degrees of skill.
Adjacent terms
An intertitle, Gérard Genette’s term, is the title of an internal division of a book, a chapter heading or a Part title. An epigraph is a quotation, real or invented, placed at the head of a chapter. A part-issue, or number, is a physical instalment of a serially published novel, holding several chapters, and a number plan is the working note Dickens kept for each monthly number, setting out its chapters and what to withhold. The feuilleton is the serial novel printed at the foot of a French daily newspaper.
What this chapter leaves to others
How the reader is carried across a gap once it is placed (anchoring, orientation, the typography of the mark) belongs to The Transition (Chapter 5). What a chapter contains belongs to The Purposes of a Scene (Chapter 1) and Scene and Sequel (Chapter 2). A change of viewpoint at a break, considered as a viewpoint decision, belongs to The Viewpoint Shift (Chapter 13); this chapter treats the handoff only as a place to cut. The first chapter of a book is the subject of The Opening Pages (Chapter 9), the prologue that of The Prologue (Chapter 10), and the last chapter that of The Ending (Chapter 16), which inherits this chapter’s mechanism at the largest scale. The alternation of subplot threads across breaks is in The Subplot (Chapter 14). Non-linear and Multi-timeline Architecture (Chapter 8) takes up timelines that alternate by chapter, and Series and Genre (Chapter 17), the break between volumes.
See also: the Reference definitions, with tables of the scales of division and of the configurations of chapter and scene.