"Science Fiction is speculative fiction in which the author takes as his first postulate the real world as we know it, including all established facts and natural laws."
- Robert Heinlein

There are earlier uses of this word in science fiction. For example, in Eric Frank Russell's 1941 story Beyond All Weapons.

You made a name for yourself in the field of Psycho-History and taught at the University of Megalon...

The Director’s eyes were dangerous. But I plunged on recklessly. “Yes, mass hypnotism — that has been your secret weapon. You may have deluded most of the world, but you forget I studied psycho-history in what you are pleased to call the Barbaric Era. Earlier dictators than you fumbled around with the idea of mass hypnosis. There was a man named Hitler back in the 1930s and ’40s who used it with some crude success. But he didn’t know anything about the scientific end of it. He employed no hypnosis rays or skillful projections to break into the unconscious and the subliminal thresholds. He relied on a raucous voice and emotional reiterations. You have been far cleverer. Your Sunday Assemblies renew your contacts with all the earth, rebathing the poor dupes in hypnosis rays and sliding afresh the projectional suggestions of your voice and mind into theirs.”

I held her tight. “All eyewitness accounts are like that,” I told her earnestly. “As the wife of a man who has devoted most of his life to the application of psychology to historical data you ought to realize that.