Not income at all which and also the reason why it’s taxed very differently (and perhaps unfairly).
Income is your wage or the actual money you make from your business. Most rich people generate wealth via their assets (e.g, the stock / property they own skyrocketed in value), and these are only taxed when sold and at a much lower rate.
There is some support for lower income brackets but my personal opinion is there should not be “billionaires” in the first place, and that’s where the system falls short. “Billions” is an obscene amount of wealth to be had by one person.
Edit: Here is a visualization on how much billions of dollars really is. Truly mind-blowing.