Disposable income is essentially just income after tax. It’s the amount you have where you get to direct where it goes / how you dispose of it. It’s not money after essentials.

There are many ways to slice it but the US median income is high.

https://en.wikipedia.org/wiki/Disposable_household_and_per_c...

TIL, I've always thought it was minus housing etc.

Apparently that's discretionary income.