Assuming I am being asked from a genre studies perspective, I like to define Science Fiction in accordance to Ursula K. Le Guin’s definition of the text we read in class:

Words can be used thus paradoxically because they have, along with a semiotic usage, a symbolic or metaphoric usage. All fiction is a metaphor. Science fiction is a metaphor. What sets it apart from older forms of fiction seems to be its use of new metaphors drawn from certain great dominants of our contemporary life—science, all the sciences, and technology, and the relativistic and the historical outlook, among them. Space travel is one of these metaphors; so is an alternative society, an alternative biology; the future is another. The Future, in fiction, is a metaphor” (Le Guin:1980)

Sci-Fi, like all great genre fiction, works on two levels simultaneously: the literal (plot-level) level, and on the symbolic (meta-story) plane. This double narrative allows the reader to engage with deeper philosophical anxieties on the metaphoric level, while being carried by an awesome plot of adventure in space with mega advanced technological reach and aliens.

Science Fiction came to exist when, through the use of science, humanity realized that the architecture of its reality was not definitive or designed, but merely circumstantial. This notion affects the human psyche in two ways: the modern human realizes how lucky he is to have such a privileged vantage point (because literally one hundred years before his own existence humanity had no idea of its own position within the cosmos). But then also his imagination immediately tortures the modern human when (s)he realizes that a more spectacular existence is not only possible, but a matter of time.

The modern (wo)man comes to the realization that our consciousness provides us with a small window to observe first-hand the spectacular timeline of human civilization and technological development, and that the future is unattainable. We know that the pace of progress is escalating at an accelerated rate because of our ability to study the past, through writing, markings, images, morphology, etc. But the future we cannot access. Not literally; we must resort to the metaphor.

That’s where Science Fiction comes in; it is that metaphor we needed to access the future. It’s not the literal future, no. You are no supposed to take it too seriously. It’s like Le Guin said: Science fiction is a metaphor. Science, the future, in fiction, are all metaphors.