Effective stack
The smaller of two players' stacks in a confrontation, since that is the most either side can actually win or lose in the hand.
A big stack facing a much shorter one is only ever playing for the short stack's chips, no matter how many more sit behind it; anything above the shorter stack is irrelevant to that particular pot. Effective stack size, usually measured in big blinds, is the number that actually decides strategy: 100 big blinds effective leaves room for a normal raise-and-play-postflop game, while 15 big blinds or fewer collapses the decision to push/fold.