Here's how I define it.

Poverty = you can't afford basic needs, like room/shelter/etc.

Upper class = you have enough that you don't ever have to worry about money to cover your costs.

Middle class = everything in between.

How do you define middle class?

ETA: I think there's an extra category (Upper Middle Class) that seems to be shrinking... it's where you still have to think about money, but you are able to save for the future and live a lifestyle where you're not constantly worried about money. That used to be pretty common but it's a lot more rare today.