Think of this as the truest child of the scientific romance of the 1800s. The science is floozy, as when a Vernian villain declaimed, “My electricity is not your electricity.” The story’s the thing, not the science. It’s softer than soft scifi.
For all that, some very scientific explanations may take over for awhile, as you explain how the radium bullets work or the plants providing oxygen scrubbing in your gravitic spaceship. But it’s really hand-waving. Whether you have an effect happen through a magic ring or “the nanobots do it somehow,” if you can’t pin it down with real science, it’s about the same.
So we have the merry realm of science fantasy, where adventure lies and derring-do, where mysteries are still solved by ratiocination, and we aren’t going to slow down to explain to much, for the same reason you keep moving fast on thin ice.
A lot of space opera falls here, as does Used Furniture that tries to transplant film noir or Western frontier stories into future worlds.
It is a wonderful blend of reality with “what if”. It differs from soft SF in that the basic premise springs from science, but tips over at some point into possibilities not driven by pure science. If your magical effects are given scientific sounding names without a good scientific extrapolation behind them, your work can be called science fantasy. Many people put tales of psychic powers in an otherwise normal world here, because they swear up and down letting parapsychology into the Academy of Sciences was just wrong.
Anything off-Earth by Edgar Rice Burroughs, but especially the Barsoomian books.
The Barsoomian trilogy is a classic worth reading: A Princess of Mars, The Gods of Mars and Warlord of Mars, as well as the other books of the series:
Thuvia, Maid of Mars
The Chessmen of Mars
The Master Mind of Mars
A Fighting Man of Mars
Swords of Mars
Synthetic Men of Mars
Llana of Gathol
John Carter of Mars
These are sometimes considered the poster children for the “sword and raygun” sub-genre. Though they didn’t have rayguns, just exploding radium bullets fired by hand-held rail guns.
The Darkover series by Marion Zimmer Bradley, including:
Heritage of Hastur
The Spell Sword
The Winds of Darkover
Fires of Azeroth, Gate of Ivrel, Well of Shiuan and Exile’s Gate by C. J. Cherryh
Rider at the Gate and Cloud’s Rider by C. J. Cherryh
The Tuesday Next series by Jasper Fforde (The Eyre Affair, et al) which uses pseudo science to bring literature to life
Pilgrimage and The People: No Different Flesh by Zenna Henderson.
The Birthgrave, Vazkor Son of Vazkor, and The White Witch by Tanith Lee.
To Ride Pegasus by Anne McCaffrey.
Moon of Three Rings by Andre Norton.
The Witches of Karres by James H. Schmitz.