The modern usage of the term “middle-class”, however, dates to the 1913 UK Registrar-General’s report, in which the statistician T.H.C. Stevenson identified the middle class as those falling between the upper-class and the working-class.[14] The middle class includes: professionals, managers, and senior civil servants. The chief defining characteristic of membership in the middle-class is control of significant human capital while still being under the dominion of the elite upper class, who control much of the financial and legal capital in the world.

Literally just the wikipedia page about the middle class lol

Yes, “middle income” is also an accepted definition, but it’s a shitty definition because it doesn’t mean anything! Income is dependent on markets as much as it is on social structures, and the “middleness” of income is as much of a feeling as it is based in literal dollars. That’s why everyone calls themselves middle class regardless of their actual place in the socioeconomic hierarchy.