Which brings me to how I've actually been defining what it means to be a superhero. Namely, that a superhero is someone who can confidently leap off the side of a tall building confident that they won't get hurt. Flight is the obvious (well, obvious in the context of comic books) solution.
"Ah, but what about Batman? Or Daredevil? They can't fly."
No, but those types of characters are both A) athletic and B) resourceful enough that they have other options to prevent them from hitting the ground with a splat. Often involving ropes and cables.
Although, it sometimes just involves being really, really acrobatic.
Another possibility, too, is that the character is just so tough that smashing on the ground is just going to put a dent in the pavement, without actually doing them any harm.
The point is that a superhero can casually leap off a building with no real concerns about becoming a puddle on the street. That's how you can lump Superman (an alien) in with the likes of Thor (a god) and Iron Man (a regular guy with lots of technology) and the Vision (an android) and Zatara (a magician) and the Phantom (just a really athletic guy) and Deadman (a ghost) and Plastic Man (an altered human).
Of course, the down side to my definition is that it also includes characters most people wouldn't normally think of as super-powered. Like Wile E. Coyote.