Science fiction (abbreviated SF or sci-fi with varying punctuation and capitalization) is a broad genre of fiction that often involves speculations based on current or future science or technology.
Science fiction differs from fantasy in that, within the context of the story, its imaginary elements are largely possible within scientifically established or scientifically postulated laws of nature (though some elements in a story might still be pure imaginative speculation).
For more content on the Science Fiction genre and its various subgenres take a look at this separate guide.