chapter title page; divisional title page
In book design, a page that introduces a new chapter or section, often with the title and sometimes decorative elements.
This book has a chapter title page before each chapter.
middle door; inner door
A door located between rooms or in the middle of a building, as opposed to an entrance door. Less common than the book-design sense.
When I opened the middle door in the hallway, there was a small garden.
Compound of 中 (naka, 'middle, inner') and 扉 (tobira, 'door, title page'). The book-design sense comes from the idea of a 'door' into a new section.