My “Word Origin Calendar” reminds me today that the term “parallel universes” was invented by an sf writer:

In 1934, a science fiction writer named Murray Leinster published Sideways in Time, a short story positing that time travelers would not experience a single past and future, but instead a series of alternate worlds existing alongside each other. He called them “universes in parallel,” which subsequent writers adapted as “parallel universes.”

Seminal Leinster stories Sideways in Time, A Logic Named Joe (which presages the age of the personal computer), and First Contact can all be found in the First Contacts: The Essential Murray Leinster, published in 1998 by NEFSA Press.