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In writing you have fiction and non-fiction. Under fiction, you have fantasy and reality. Reality is a story that could conceivably happen in our world/reality. Fantasy is anything that couldn't...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/48969 License name: CC BY-SA 4.0 License URL: https://creativecommons.org/licenses/by-sa/4.0/
#2: Initial revision
In writing you have fiction and non-fiction. Under fiction, you have fantasy and reality. Reality is a story that could conceivably happen in our world/reality. Fantasy is anything that couldn't or hasn't happened. Within fantasy, you have SciFi, Magical, Alternate reality, etc. Within reality, you have mysteries, westerns, romance, etc. To me, the true genre bender is horror. Some people believe in ghosts, monsters, demons, etc. and other don't. I suppose the same could be said for aliens. All that to say, don't try to categorize your story so much as try to tell your own unique story. Let the bookstores and publishers decide where to put it for the best marketing. So yes, your story would be fantasy as long as there is something about your world that makes it not ours. New races, different life span, strange physics, anything is possible and without magic being required.