Wealthy -- I don't have to work to maintain my lifestyle.
Poor -- I can't afford basic necessities even when I work.
Middle-class -- everyone else.
Now for sure there is some variation on middle-class (upper/lower/truly middle) and that's basically a function of disposable income.
But this definition at least accounts for variation in regional cost of living and even adjusts for personal lifestyle choices. Your middle class might be my wealthy because my lifestyle is cheaper.