I propose the following distinctions:
Science Fiction: Fiction with science and/or technology elements that, at the time of their writing, either match what we know about science, or aren’t ruled out by what we know about science. Ex: “Lucifer’s Hammer”; “The Two Faces Of Tomorrow”; “Neuromancer.”
Speculative Fiction: Fiction with science and/or technology elements that don’t match what we know about science, or are ruled out by what we know about science. Ex: “Star Trek”; “Firefly/Serenity”; “Star Wars.”
Fantasy: Fiction entirely unrelated to our reality, no matter how far you’re willing to stretch science. Ex: “Lord of the Rings”; “A Voyage to Arcturus;” “A Wizard of Earthsea.”
SyFy: Pitifully incompetent examples of any of the above three. Example: What Hollywood turned Harry Harrison’s excellent SF novel, “Make Room, Make Room”, into: “Soylent Green”; “Dr Who” in any of its manifestations; “Lost in Space.” When you say SyFy, you’re required to sneer.