I've been asked several times now what would be considered 'sci-fi'. The simple answer is science and fantasy. The long answer can be found on Wikipedia, I've linked the subgenre's below so you can look through and get clarification, inspiration and get the juices flowing!

Via Wikipedia, the definitions are as follows:

Science fiction is a genre of fiction dealing with imaginative content such as futuristic settings, futuristic science and technology, space travel, time travel, faster than light travel, parallel universes, and extraterrestrial life.

Categories of sci-fi are as follows:

Hard science fiction, or "hard SF", is characterized by rigorous attention to accurate detail in the natural sciences, especially physics, astrophysics, and chemistry, or on accurately depicting worlds that more advanced technology may make possible.

The description "soft" science fiction may describe works based on social sciences such as psychology, economics, political science, sociology, and anthropology.

Source: Wikipedia