What is sci-fi? Two answers:
a - it's a genre and is therefore defined in a circular way as being the books that are enjoyed by sci-fi fans, or written by sci-fi authors, or found in the sci-fi section of your local bookstore. The idea of what makes something sci-fi can change over time, and of course there is the possibility of disagreement or different perspectives.
b - John W. Campbell had this idea that a true science-fiction story could only have one difference from the real world - this could be a divergence from history (a Jonbar point) a change to the laws of physics, or some other invention or postulate; the story should explore the consequences of this one change. This makes science-fiction similar to the idea of gedanken-experiment (thought-experiment).
I prefer b personally: this excludes anything in the Star Wars / Star Trek area (fun though they are) and relegates them to fantasy, while confirming the status of Woody Allen's The Purple Rose of Cairo as classic science-fiction.