First time I heard FAT FIRE and THIN FIRE. What are they?

Lean FIRE is when someone has saved up 25 times their annual expenses and lives on a "lean" budget, spending less than the average American (around $60,000 a year).

By contrast, someone who achieves Fat FIRE spends more than the average person. Pursuing a Fat FIRE number — regardless of how much you actually plan to spend — can afford greater flexibility, freedom, and protection in early retirement.

With Coast FIRE, you save to a target number by a certain age and then stop saving and let compounding gains "coast" you to your ultimate target retirement nest egg.