Riddle answer: pension (or other fixed income) that covers expenses.
You could have a mortgage of 500k, credit card debts of 20k, and no assets, and as long as your income covers all those expenses, and your other living expenses such as food (and, hopefully is COLA'd to cover inflation), I'd consider you FI, even with a negative net worth and a mortgage and no assets.
Nords (author of The Military Guide to Financial Independence and Retirement) is a great example though of someone who has a military pension and doesn't have much need for more money. Now sure, he has other assets (isn't net worth negative, like in the extreme example), but his pension allows him to donate all the proceeds from his book and blog to military charities and his wife to turn down high CEO-level compensations and work in the same job for free. And it even allows him to invest fairly aggressively, from what I understand, and keep a mortgage. In fact, although he could pay off the mortgage, he's an example of someone keeping it to invest the amount and try to earn a spread (and, I believe, took out the mortgage on a paid off property - or one that he could pay off - specifically for this purpose).
:)