From Latin 'capitulum' (little head) — each text section had a heading, and monks' daily readings became 'chapters.'
A main division of a book or other written work, or a distinct period or sequence of events in a person's life or in history.
From Latin 'capitulum,' a diminutive of 'caput' meaning 'head.' The original sense was 'little head' — a heading or title at the top of a section of text. In late Latin, monastic communities used 'capitulum' for the passage of their Rule read aloud each day, and then for the assembly at which it was read, giving rise to the ecclesiastical sense of 'chapter' as a body of canons or monks. The word entered English via Old French 'chapitre.' Key
A 'chapter' is literally a 'little head' — from Latin 'capitulum,' the diminutive of 'caput' (head). Medieval monks would gather each morning to hear a 'capitulum' (a chapter of their Rule) read aloud, and the meeting itself became known as a 'chapter.' This is why a group of cathedral canons or monks is still called a 'chapter,' and the room where they meet is a 'chapter house.'