Effective stack
The smaller of the two stacks in a pot — the most either player can actually win or lose.
Say the button opens A-K and has you buried, sitting on 200 big blinds. You're in the big blind with 8-8 and 60bb behind. The hand plays as a 60bb pot, not a 200bb one. The button's extra 140bb never enters the math, because you can't lose more than your own stack and the button can't win more than you can put in. Flop comes 8-4-2, the money goes in, you stack off 60bb and double up. The tower in front of the button is decoration.
That number drives real decisions. Stack-to-pot ratio, whether top pair is worth stacking off, whether the all-in is a coinflip or a mugging — all of it measures against the effective stack, not the bigger pile. A short effective stack turns marginal hands into shove-or-fold. A deep one buys room for multi-street plans, slowplays, and bluffs that need a runway. Watch the trap here: being covered is not the same as being deep. If the smaller stack is short, nobody has room to maneuver after the flop, no matter how much sits behind.
The common leak is sizing bets and reading commitment off your own stack instead of the smaller one. A player with 300bb facing a 20bb short stack still only wins or loses 20bb in that pot — the other 280 sits idle. It also shifts hand to hand. Open for a raise with 100bb, get called by a 40bb stack and a 100bb stack, and you're playing two different effective stacks in the same pot.