What Makes something Sci-Fi

I think Sci-Fi is considered Sci-Fi because well its stuff that just doesnt seem possible. Aliens, Space battles, Ufo's all of those things just dont seem like they could ever in a million years be real to humans. I think the fact that most of it take place in space also is a huge factor in alot of sci-fi. There are some things that take place on earth too which makes it seem as though its possible like one day aliens could actually come down from space and attack and kill us all. I think those are the main components of Sci-Fi.