FI = Financial Independence = bare minimum to FIRE = 25x expenses
FU = Enough money to no longer be afraid of losing your job = ~ 1 year of expenses (or however long to comfortably get a new job).
I totally agree with this. I'm in a bit of a rough patch with my job, which I post about probably too much in another thread, but one thing that makes me sleep okay is knowing that we have enough saved up such that if I were to become unemployed (either by choice or not), I'd be just fine.
My wife makes enough income to cover all bills, which is a tremendous security blanket. On top of that, we have about $20,000 in cash, another $12,000 in Roth contributions (which we could pull at any time without penalty), and then a bunch of money stocked away in retirement accounts. Obviously we don't want to tap into those, but those are at least there.
Bottom line, I think FU money is being able to walk away from your job and have finances be pretty low on your list of concerns. I would imagine that's where you're at if you're this close to FI. If not, start saving some cash reserves.