Is a "secondary" universe a "parallel" universe? Or not?

"Secondary world", as I've heard it used, is an out-of-universe descriptor of a setting. It basically means to the potential reader that the author has written a story in a fictional setting.

Whereas parallel worlds are an actual (theoretical) scientific concept about other worlds existing alongside our own, often used in the context of people from our world traveling to them.

Long story short, one is a literary genre description and the other is a scientific concept.

Agreeing with Margrave86.

I tend to see "secondary" in the context of "totally made up, possibly inspired by Earth but intended as an unconnected fiction." It's a "you can't get there from here" world.

"Parallel", at least to me, is either a tangible offshoot of Earth - say, where dinosaurs never went extinct, or where dragons are real - or a world somehow connected to our Earth: an alternate reality or timeline, etc. Some are so far removed they're essentially secondary worlds, but there's often some point of contact, some hint of a shared origin or destiny. Since one can't be parallel without a thing to be parallel to, the term tends to lack meaning if there's no communication between "our" world and theirs, though sometimes it's a set of multiple "layered" realities, none of which are quite ours (like V. E. Schwab's multiple Londons in her Darker Shade of Magic series; even the one most like ours has a tinge of magic that our world lacks, but the three Londons are parallel worlds related to each other.)

Not sure if there's an official or "industry standard" definition, but as a reader that's how I usually see them used.

Hi there! To keep this short & sweet, I honestly think it may well depend on the context, though as an alternate history writer myself, I've tended more towards the view that secondary universe = parallel universe, by and large.

Not sure secondary world was used before this: Secondary world is a term used by Tolkien to refer to a consistent, fictional world or setting, created by a man, also called subcreation, in contrast to the Reality, called Primary world.

Tolkien claimed that the author should respect his creation and grant it internal consistency, and let it obtain 'life' of its own. The tales should have several dimensions: geography, characters, languages, timeline, all being inter-dependent. The "scenery" should seem able to sustain the events and characters it hosts, and this would make the effect credible to the reader.

Nowadays the concept is better known as conworld or fictional universe.

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When I think of a parallel universe the multiverse theory comes to mind. In fiction, I've never been impressed by the concept that everything is the same except X. While it makes sense that with infinite possibilities there is indeed the one universe where everything is the same except X, how would you find that one universe?

Consider the concept, X is different. Okay but everything from there forward is going to spread out in different directions. How did you come across that one universe just after X changed. Does that mean if you had come across it a couple years earlier everything would be exactly the same?

It's great for fiction to explore concepts like Hitler winning the war or whatever. But my logical brain simply cannot buy it if you try to make it anything but completely fictional. In other words, don't try to explain it as a parallel universe. If, on the other hand, you want to write about the fictional world where Hitler won the war, go for it. Just don't try to fit it into the illogical conditions that could arrive in a parallel universe.

Not that the reader would notice or have that incessantly logical need that I have.

Now onto the actual scientific theories of the multiverse world. There are theoretical physicists like Brian Greene that are serious about such a condition.

A Physicist Explains Why Parallel Universes May Exist

Earlier components like 'branes' which are the membranes where multiverses bump up against each other were theorized decades ago. Brane cosmology (Not the best source but better ones are uselessly over-technical.)

MaeZe said:

When I think of a parallel universe the multiverse theory comes to mind. )

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Exactly. The parallel universe / multiverse theory comes from quantum theory.

Schroedinger's cat. Quantum particles behave differently if observed vs unobserved