Is there a term for the opposite of anthology?

If you define "anthology" as multiple works of one or more authors in a single volume, then what do you call a single work that is published in multiple volumes?

"Lord of the Rings" is a similar case to this situation, but not really apt, because each volume of the 3-volume work has a unique title that it was published under.

Specifically, I have "The Decline and Fall of the Roman Empire" (Gibbon) in 2 editions: the first is a 2-volume edition, and the second is a 6-volume edition. The individual volumes are basically titled: "Volume 1", "Volume 2", and so on.

The reason I ask is that I’m putting together a desktop/web app to classify and organize my home library, and I have terms for the other common concepts, aside from "anthology":

"series" — open-ended list of related titles by the same author(s)
sharing a story arc

"set" — close-ended and named collection of works (a.k.a "The Great
Books of the Western World")

I’ve looked and searched pretty widely, but I can’t find a term for a single work in several volumes. The most common alternative I’ve seen is "multi-volume set", but that doesn’t quite cut the mustard.

The closest thing I’ve been able to find is the triple-decker format from the Victorian era when novels were often published in 3 parts.