Depends who you ask, or the context. Anecdotally, software developers, being more math-minded than average, tend to think of middle-class literally - the middle third or middle quintile of income distribution.

That has never been the meaning of the term historically and doesn’t capture the magnitudes-difference between the middle and upper classes.

I agree. I was just pointing out that there is a portion of the HN readership that tends to use the strictly economic definition (rightly or wrongly). Also, the meaning has shifted over time.

As used in the early 1900s - I realize the term was coined even earlier than that - it referred to what today we'd likely call upper-middle-class (or if you're a fan of Engles, the bourgeoisie). White-collar, professional, well-educated, but not rich/powerful/nobility. It excluded almost the entirety of the working-class (even those who, by income, were well above poverty).

More recently, usage in the US has trended towards anybody above poverty but not quite rich (and choose your own definition of rich to suit your point). Which itself includes a massive span of incomes and lifestyles.

The usage hasn't really changed in the UK.

depends how you slice it. "have to work" is kind of a vague way to define class membership. someone with a $500k net worth doesn't "have to work" if they're content with living on $20k/year or so. of course, if you insist on sending your children to private school and taking them to europe every year, this is going to be unworkable.

most people tend to think what they have isn't quite enough, twice as much would be just right, and anything more than that is excessive.