Speculative fiction that asks the question: What is the role of human beings in a universe without gods or magic?
Many people think sci-fi is all space opera but that's only a subgenre. The very first work of sci-fi was Mary Shelley's Frankenstein, which was written at the end of the Enlightenment and the beginning of the Industrial Revolution, a period when science had freed people of the notion that there was any divine or supernatural influence on their lives and at the same time giving them extraordinary powers over nature. Tasked with writing a "ghost story," Shelley's genius was to dispense with the supernatural element altogether and ask whether humans could become godlike themselves, even to the extent of actually creating life. With the Industrial Revolution in full swing, sci-fi began to focus on machinery, especially transportation and the wonders it could reveal, so you get Verne's subs, balloons, and rocket ships. This element of wonder, once the province of fantasy, became especially emphatic when one considered that sci-fi's marvels could actually be real someday.