Wikipedia on Science Fiction: "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. It often explores the potential consequences of scientific and other innovations, and has been called a 'literature of ideas'."

There are some who consider Gulliver's Travels to be one of the first imaginative. Frankenstein is another early example of the genre.  More generally Jules Verne and H.G. Wells have often been cited as fathers of the science fiction genre.