Is a Novel a series of Chapters, or are chapters just digestible parts of a novel?

Tolkein wrote The Lord of the Rings as one complete work. It was subdivided into three “books” by his publisher. When I am drafting a novel (or novella, as I currently am), I usually start with a chapter number in mind (The Sword started as 12 chapters, then became 15). A chapter is a kind of movement, or act, with a beginning and an ending. Now this isn’t rigid, and sometimes a chapter has to be further divided or combined with another. But I have an idea of when its done when I start writing it.

Currently I’m reading Joan Didion’s Play It As It Lays, and she does that amusing thing, that I haven’t seen since Douglas Adams, of having chapters that are barely more than a paragraph, while others go on much longer. These are chapters that simply denote the passage of time, and some can be mere snapshots of a moment that moves the characters or underlines a point about them. A chapter can be literally whatever you want a chapter to be.

Hence my question.