Story structure
Chapter
Any of the main thematic divisions within a writing of relative length, such as a long work of prose or poetry.
A chapter (capitula in Latin, sommaires in French) is "any of the main thematic divisions within a writing of relative length." The division is not natural or ancient: many ancient texts ran unbroken, and numbered chapter divisions spread from about the fifth century, Jerome's capitulum an early use for numbered headings. Chapters are usually numbered sequentially and may carry titles, epigraphs or contents summaries, and some books play with the numbering itself: prime numbers only in The Curious Incident of the Dog in the Night-Time, thirteen chapters always in A Series of Unfortunate Events.
For serial writers the chapter is the atom of the craft, the unit of drafting, release, payment and reading. Its length is a planning decision with schedule and money attached, weighed in how long should a web novel chapter be, and its ending is the hook that carries readers across the publish gap, covered in cliffhangers and chapter endings.