The term "sci-fi" was publicly adopted as an alternate form of "science fiction" by Forrest ("Forry") J Ackerman by 1954, in analogy to the then-cutting edge "hi-fi".
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Ackerman was a long-time fan who created Famous Monsters of Filmland magazine, which celebrated the monster/SF movies that were frowned upon by many hardcore science fiction fans as being too poor in quality to be taken seriously, and definitely not to be confused with "real" science fiction.