Bankroll
The money set aside exclusively for poker, separate from rent and bills.
Your bankroll is the dedicated pool you play poker with. Not rent. Not "fun money". A separate balance you can track to the cent at any moment.
Standard bankroll requirements (BRM): 20–30 buy-ins for live NL cash, 30–50 for online cash, 100–200 for MTTs, 200+ for hyper-turbos and Spin & Go. These numbers are calibrated to the typical standard deviation of each format — they're not arbitrary.
Move-down rule: pre-commit a stop. "If I drop X buy-ins from my peak, I move down a stake." Emotional move-downs after a bad session are usually too late.