The [Pew Research article they linked to](https://www.pewresearch.org/fact-tank/2018/09/06/the-america...) uses this definition:

> In our analysis, “middle-income” Americans are adults whose annual household income is two-thirds to double the national median, after incomes have been adjusted for household size.

And they seem to use "middle-income" and "middle-class" interchangeably.

Note that this definition is quite different than what may have been used in your history/economics classes in school/college.