I'd say middle class is anyone in the range of income/wealth that makes the following statements true:

- does not need to worry about having enough money to afford basics like food/shelter/healthcare/etc on a regular basis. That is, they make enough to have security in their access to basic needs.
- does not have a store of wealth sufficiently large that earning income is no longer necessary to ensure the above security for the rest of their life

In the US, anyone making more than about $20-50k/year (depending on location) and holding less than about $2 million in liquid assets could fairly be characterized as middle class.

If you start earning $250k you're middle class for a while, until you've accumulated a few million in your savings/stocks/bonds/etc. Once you have that few million in liquid wealth (excluding value of primary residence), you're upper class, I'd say. If you're earning $500k but spending it all and never manage to accumulate any wealth, you're probably still middle class since if you lose your job you're just as screwed as the guy making $50k. Upper class is about having the freedom to not need to work for a living any more.