A novel sequence is a set or series of novels which share common themes, characters, or settings, but where each novel has its own title and free-standing storyline, and can thus be read independently or out of sequence.
There is no useful, formal demarcation between novel sequences and multi-part novels. Novels that are related may or may not fall into a clear sequence. It is also debatable whether a trilogy is long enough and whether its parts are discrete enough to qualify as a novel sequence.
For example, the Barchester novels of Anthony Trollope are only loosely related, although they contain a recurring cast of characters; his political novels about the Pallisers have a tighter connection and dynamic. A strict definition might exclude both.
The novel sequence was a product of the nineteenth century, with James Fenimore Cooper's works appearing in the 1820s, and Anthony Trollope's Barchester books in the 1850s. In French literature, Honoré de Balzac's ambitious La Comédie humaine, a set of nearly 100 novels, novellas and short stories with some recurring characters, started to come together during the 1830s. Émile Zola's Rougon-Macquart cycle is a family saga, a format that later became a popular fictional form, going beyond the conventional three-volume novel.
A roman-fleuve (French, literally "river-novel") is an extended sequence of novels of which the whole acts as a commentary for a society or an epoch, and which continually deals with a central character, community or a saga within a family. The river metaphor implies a steady, broad dynamic lending itself to a perspective. Each volume makes up a complete novel by itself, but the entire cycle exhibits unifying characteristics.
The metaphor of the roman-fleuve was coined by Romain Rolland to describe his 10-volume cycle Jean-Christophe. In the preface to the seventh volume, Dans la maison (1908/1909) he wrote: "When you see a man, do you ask yourself whether he is a novel or a poem? ... Jean-Christophe has always seemed to me to flow like a river; I have said as much from the first pages." The term has subsequently been applied to other French novel sequences, particularly of the years between the world wars, notably: