Business Insider had an article defining the "undefined economic anxiety of the middle class".
If you are in the middle class, 70% of your expenses go to housing, education, healthcare and food. All of those categories are going up at faster than the rate of inflation. And the official rate of inflation is 2% while rejecting the idea that people substitute cheaper stuff when prices go up, real inflation is 3%.
When you have flat wages accounting for 2% inflation and -1% per year with real values, while the biggest expenditures people pay has gone up faster than inflation accounts for the economic stress - the economic squeezebox.
If you are in the lower class, Business Insider said 90% of a poor person's income went to those categories. And those in the upper class spent less than 40% of their income on those categories.
So the dividing line would be how much of one's income goes to those four categories, and thus the stress their rise in prices causes.