To expand a bit, it seems many people still use the definitions of "fantasy = magic" and "sci-fi = futuristic technology", which I find rather strange because those definitions have never really held up. We've always had fantasy stories that involved futuristic technology and sci-fi stories that involved magic.
The definition I usually use is that a fantasy world is a world that operates on different laws of physics than ours - and as a result has different intelligent species, different history, different societies etc.
If it's a different history and society but all the laws of nature are the same as in our world, then it's just alternate history fiction.
A sci-fi world on the other hand is a world that shares the real world's history up to the current time and speculates on how it could go further into the future.
It blends ofcourse, when the base of your world is "thousands of years into the future, humanity has settled on a world with very different laws of physics and forgotten earth completely".
The definition I usually use is that a fantasy world is a world that operates on different laws of physics than ours - and as a result has different intelligent species, different history, different societies etc.
If it's a different history and society but all the laws of nature are the same as in our world, then it's just alternate history fiction.
A sci-fi world on the other hand is a world that shares the real world's history up to the current time and speculates on how it could go further into the future.
It blends ofcourse, when the base of your world is "thousands of years into the future, humanity has settled on a world with very different laws of physics and forgotten earth completely".