Science Fiction is the genre which, when you try to define it you fail.
I think part of the trouble with making a definition of Science Fiction is that the border between Fantasy and Science Fiction is a lot blurrier that writers of definitions like to believe, especially since a lot of them seems to make the definition for the specific purpose of distancing themselves from Fantasy.
To actually make a definition I'd use two steps, first an inclusive step trying to defined oth as one, then define the difference, so:
Fantasy and Science Fiction is a genre of stories taking place under the constrains of a world where the rules are different.
Science Fiction is the sub genre where the rule change is limited to the scientifically plausible.
For both SF and F a major requirement for quality is that the rule changes are consistent and pertinent .
Reductionist and proud of it.
Being ignorant is not so much a shame, as being unwilling to learn. Benjamin Franklin
Chase after the truth like all hell and you'll free yourself, even though you never touch its coat tails. Clarence Darrow
A person who won't read has no advantage over one who can't read. Mark Twain