The word science-fiction is often abbreviated in SF or even Sci-fi, refers to a genre of literary fiction. Most of the time a SF novel or short story involves speculations based on current or future science or technology. There are many sub-genre connected to the science-fiction itself that is why it is difficult to define clearly what is science-fiction.

Among these subgenres we can quote Time Travel, Cyberpunk, Alternate History, Military SF, Soft and Social SF, Hard SF, etc.
Science fiction is largely based on writing entertainingly and rationally about alternate possibilities in settings that are contrary to known reality.
This genre was especially famous in several media in the seventies and eighties for example with some TV shows like Star Trek. Science-fiction implies imaginative elements not found in contemporary reality.

SF genre is often assimilated to the fantasy genre; but the SF differs dramatically from it, indeed its imaginary elements are largely possible within scientifically established or scientifically postulated laws of nature (see Isaac Asimov novels).
The main goal of the science-fiction genre is to explore the consequences of technologies on the society or individuals (see for example Brave New World); main topic can be the following setting in the future (taking place in different time lines); setting in outer space, on other worlds involving aliens sometimes; story involving technology or scientific principles (in contradiction with known laws of nature); story involving discovery or application of new scientific principles (time travels, nanotechnology, human-like robots, or new and different political or social system and so on.).