WordNet records no synonym and no antonym for chapter. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a subdivision of a written work; usually numbered and titled
“he read a chapter every night before falling asleep”
any distinct period in history or in a person's life
“the industrial revolution opened a new chapter in British history”
a local branch of some fraternity or association
“he joined the Atlanta chapter”
an ecclesiastical assembly of the monks in a monastery or even of the canons of a church
a series of related events forming an episode
“a chapter of disasters”
Synonyms, antonyms and the broader and narrower relations for chapter come from Princeton University’s WordNet. Full sources and licences.