In the famous schoolroom scene that opens Charles Dickens's Hard Times (1854), the pedantic Mr. Gradgrind asks the novel's heroine to define a horse. Although she has spent her life around circus ponies, Sissy Jupe is struck dumb. Her horrible classmate Bitzer supplies the answer:

Dickens wants us to feel that this definition is as wrong as it is right, that for all its factual precision it doesn't really bring us much closer to apprehending the object of study. After all, Sissy the circus girl knows horses far better than Bitzer does, but she couldn't care less about grinders, incisors, and the shedding of hooves in marshy countries.

This is the risk you run when you try to define the novel. In response to the Gradgrind imperative to begin by saying what a novel is, we might propose something along these lines: “A novel is a self-contained piece of fictional prose longer than 40,000 words.” There are famous exceptions to this definition – Pushkin's Eugene Onegin (1833) is a novel in verse, for example, while the modern “non-fiction novel” pioneered by Truman Capote tells you that a novel needn't even be fictional.