A meaty small book could be filled with definitions of science fiction and expositions of them. The reason for this plethora of definitions is that none wholly satisfies.
The one that comes closest to acceptability is Damon Knight’s apothegm that “Science fiction is what we point at when we say, ‘This is science fiction’.”
We all agree with this, however much we might disagree with the list of things Knight would point at. We each have our own lists.