transclusion
Including part of another document in a way that is usually transparent to the end user.
Ie. all chapters of a book exist as independent documents, but they could be transcluded into a single document called ‘the book.’
In the same way, a website’s content could be structured into chunks, letting you compose ‘pages’ with relevant information where needed, instead of keeping in line with a strict hierarchy.
See the Wikipedia article about transclusion.