As a literary genre science fiction is notoriously hard to pin down with just one definition. For the layman any short story or novel with spaceships and little green men on the cover is more than enough to classify it as science fiction. However to those who are familiar with the genre, these are conventions that have not been connected with science fiction as a whole since the 1950s. The field has grown in such a way that it would be terribly unfair to still consider it as the juvenile fiction of before.

Even those in the field have many and differing ways of Brian W. Aldiss

Science fiction is the search for definition of man and his status in the universe which will stand in our advanced but confused state of knowledge (science), and is characteristically cast in the Gothic or post-Gothic mould.

Robert A. Heinlein

A handy short definition of almost all science fiction might read: realistic speculation about possible future events, based solidly on adequate knowledge of the real world, past and present, and on a thorough understanding of the nature and significance of the scientific method.

To make this definition cover all science fiction (instead of "almost all") it is necessary only to strike out the word "future."

Damon Knight

What we get from science fiction---what keeps us reading it, in spite of our doubts and occasional disgust---is not different from the thing that makes mainstream stories rewarding, but only expressed differently. We live on a minute island of known things. Our undiminished wonder at the mystery which surrounds us is what makes us human. In science fiction we can approach that mystery, not in small, everyday symbols, but in bigger ones of space and time

Theodore Sturgeon

A science fiction story is a story built around human beings, with a human problem and a human solution, which would not have happened at all without its scientific content.

Isaac Asimov

Modern science fiction is the only form of literature that consistently considers the nature of the changes that face us, the possible consequences, and the possible solutions.
That branch of literature which is concerned with the impact of scientific advance upon human beings.

As a genre science fiction is fraught with as many challenges