Assets (cash, securities, real estate, etc) minus liabilities (loans and other debt).
Many people in their 20's and even 30's (a lot on here) are in negative territory. If you rent, you probably own a car, have a little cash in the bank, and may have a meager 401k. Subtract from that your student loans, car loan, credit card debt, etc, and you probably have red ink. If you own a house you may be doing a little better, but with the drop in housing prices, you probably don't have much equity.
I'd guess 90% or better here have a net worth less than $50,000, and as many as half have a negative net worth.
OK, so assets would also include real estate, the owner actually lives in and it also includes company shares? If someone is called a "millionaire" in the US, does that refer to networth?
I would rather add up everything I can easily make into cash. This would not include (reasonable) real estate I live in and also no company shares that only have a theoretical value (no potential buyer known).
Otherwise, if you're an entrepeneur, it's quite easy to be a "millionaire". It just depends on the evaluation method of your company's value, which in real life could be 0 because you would never find a buyer.