I define it as smarter than humans. The same way a dog will never understand L'Hôpital's rule, there are probably things that human beings will never understand but future AIs will. I'd call those AIs superintelligent.
- their eyesight is too poor to read
- their paws are not designed for fine manipulations so they cannot write or type
- their throats and mouths are not nearly as nimble as ours, so they cannot vocally communicate detailed information
Even if there was an Newton-level dog, they wouldn't be able to access the ideas of an earlier Euclid-level dog. Human knowledge is not just about our big brains, we've developed many physical features that make transmission of information far easier than other species.
OTOH dogs do have a good intuitive "common-sense" understanding of arithmetic, geometry, and physics. It is the unique gift of humans that we can formalize and then extend this intuition, but this ability (and intelligence as a whole) relies on nonverbal common sense.
That said, another major difference is psychology. Switching animals, it seems plausible to me that chimpanzees are theoretically capable of doing basic calculus as a matter of pattern-matching. But you can't force them to study it! Basic calculus is too tedious and high-effort to learn for a mere banana, you need something truly valuable like "guaranteed admission to the flagship state university" for human children to do it. But we don't have an equivalent offer for chimps. (Likewise an Isaac Newton - level dog might still find calculus exceptionally boring compared to chasing squirrels.)
a human who is born blind and severely paralyzed (so they cannot speak or sign)Exactly! If a dog invented a dog superintelligence and it discovered calculus, the dogs would never understand that discovery. I think a superintelligence we build will discover things we cannot understand.