Many criteria have been suggested for defining exactly what the novel is. So varied are its manifestations that it is difficult to encompass the works of, say, Fielding and Knausgaard in a single category. After wrestling with the question for a while, one is tempted to conclude that it's a novel if you say it is. Milan Kundera comes as close as anyone to a useful definition:

A novel is a long piece of synthetic prose based on play with invented characters. These are the only limits. By the term synthetic I have in mind the novelist's desire to grasp his subject from all sides and in the fullest possible completeness. Ironic essay, novelistic narrative, autobiographical fragment, historical fact, flight of fantasy—the synthetic power of the novel is capable of combining everything into a unified whole life the voices of polyphonic music. The unity of a book need not stem from the plot but can be provided by the theme. (Shop Talk, Philip Roth, p. 94)