If I asked you to go out around midnight, you would not hesitate to come back and tell me that it was night.  Likewise, if I asked you to go out around 9 or 10 PM, or 3 or 4 AM. But the closer to sunrise or sunset it was when I asked you to go out, the more you might hesitate about saying that it is night.  You'd probably have some trouble stating the exact time that it became or stopped being night.

Many definitions seem to have a twilight zone around them unless we make some arbitrary definition, like Webster's first definition of "night."

We tend to fall back on definitions like Supreme Court Justice Potter Stewart's famous definition of "pornography" in 1964 - "I know it when I see it."

My "Webster's" defines "science fiction" as "highly imaginative novels and stories about some projected, often fantastic, scientific development.

Sometimes definitions state that the story is based on scientific facts.

But most of the time we rely on a Justice Stewart's type of definition when we decide that a story is or is not science fiction.