The term sci-fi or scifi is often regarded as an abbreviation for science fiction.  There is sometimes a connotation with the term sci-fi as being 'less serious' or less strict/developed science fiction

It may be sci-fi as a term is more general and it simply include story lines that deal with elements of technology and fantasy rather than speculative fiction. 

Movies such as star wars could be considered sci-fi but more strictly as fantasy science fiction because of its use of starships and a variety of planets.