I'd say "Class" can be used to refer to the types of economic problems or challenges facing people, regardless of salary.  In this case, if the question is "what's the best way to pay for my daughter going to an Ivy" and not "how on earth can we afford this", they're not middle-class.  If mortgage is the topic and "how are we going to ever pay this off" isn't the question, you're not lower-class.<p>I'll certainly agree that the lines are far from clear-cut, but I think it's best defined in a problem-oriented manner.  The middle class gets these tax breaks because they can't otherwise afford to send their kids to good schools or own houses, while the upper-class can.  They're facing different problems, and so the solutions are tailored differently for them.  This causes confusion when you have income-to-SoL disparities in the country ($60k goes further in Kansas than in NYC), so people at different income levels can be facing the same problems.