le site sfsignal.com a demandé à une bordée d'auteurs leur définition de la science fiction. Parmi le premier lot d'interviewés Alastair Reynolds, Karl Schroeder,Jeff VanderMeer, Nancy Kress, John Scalzi, Adam Roberts et Paul Di Filippo.

Tout est ici

Allez je vous en mets deux :

Alastair Reynolds
Science Fiction is fiction set in a future which is not inconsistent with our present knowledge of the world, or such knowledge as it exists at the time the work was written. In other words, there must be a logically-consistent roadmap between the present and the future. The future may be the moment immediately after the present, or an arbitrarily distant era. Alternate histories are not therefore science fiction, nor are fantasy works incorporating science fictional tropes. Science fiction works may come to resemble alternate histories or fantasies as they become invalidated by historical developments, but since such works were not intentionally written as AH or fantasy, they are still to be considered science fiction.

John Scalzi
I answered this for my non-fiction book The Rough Guide to Sci-Fi Movies. There, I said something is science fiction if at least one of the following three conditions are met:

1. It takes place in the future (or what was the future at the time of writing);
2. It uses technology that does not exist (or didn't exist at the time of writing);
3. The speculative elements of the story largely have a rational rather than magical basis.

This is a fairly inclusive definition.

There are lots of folks who will include puffery about how SF is a true literature of ideas, etc., but it's fairly obvious there's a goodly amount of brainless science fiction about, so let's not pretend science fiction is somehow privileged in this regard. On a practical level, what defines SF are the three elements above; what makes SF good or bad or a literature of ideas is the intent and skill of the writer him or herself.