it's sort of a tautological question. "upper class" is just terminology; it means whatever is convenient shorthand for the discussion. you and I get to decide what the defining characteristic of "upper class" is for the rest of this thread.
that was a bit snarky so I'll expand a little. I think people typically use "upper class" to signify the highest echelon in the power structure of society. but there are a lot of ways to have power over others, and it doesn't map perfectly onto money. an influential media personality may have more influence than someone far richer but unknown to the public. someone with a high worth on paper might have liquidity issues that force them to be careful with their spending. you might be wealthy and politically connected, but excluded from certain social circles anyway.
that was a bit snarky so I'll expand a little. I think people typically use "upper class" to signify the highest echelon in the power structure of society. but there are a lot of ways to have power over others, and it doesn't map perfectly onto money. an influential media personality may have more influence than someone far richer but unknown to the public. someone with a high worth on paper might have liquidity issues that force them to be careful with their spending. you might be wealthy and politically connected, but excluded from certain social circles anyway.