Explanation of My Definition
* Story may be true such as story of a past event or stories of bible, quran, ramayan etc but a SF story always be fictious, so I add the term imagination
*Creature means any object of the universe such as human beings, dinasaurs, planets, stars, galaxies, alience etc.
*Story of creatures means story does not involve God as Hindu mythological stories etc. They are not SF
*SF contains imaginations that must be scientifically justified. You cannt imagine time traveller unless it has some physical theory for its possibility.
*SF some times may not be futuristic.It may tell past e.g. jurassic park
*SF does not involve problems always. It may be a love story or story of progress.