Science fiction is a genre of fiction with imaginative but more or less plausible content such as settings in the future, futuristic science and technology, space travel, parallel universes, aliens, and paranormal abilities. Exploring the consequences of scientific innovations is one purpose of science fiction, making it a "literature of ideas". Science fiction has been used by authors and film/television program makers as a device to discuss philosophical ideas such as identity, desire, morality and social structure etc.Emphasis is mine. Speculative fiction incorporates scientific elements that don't really exist yet. If you're writing about a man who can travel through time using a machine or a drug, it's science fiction. If you're writing about a man who can travel through time by using his mind or waving a magic wand, it's fantasy.
But scifi does encompass a lot of different elements. It can range from a story about robotic warriors battling aliens on a spaceship to a story about a modern-day woman who falls in love with a man traveling through time. That's more what's come to be hard and soft science fiction. We'll talk more about that later.
Still, there are authors like Margaret Atwood and Kurt Vonnegut, who write about futuristic dystopias and time travel, but refuse to call their work "science fiction." I find a reluctance in some authors to define themselves as scifi is more about the author's reluctance to pigeonhole themselves than about the genre itself.
What do you think? Can you define science fiction? Or is it a term that has no meaning?