Effective stack

In big-bet poker, an effective stack (or effective stack size) is the smaller of the stack sizes between two people. The term reflects the fact that, when a larger stack goes against a smaller stack, only the amount of money in the smaller stack is really at stake. For example, if a $100 stack pushes all-in and is called only by a $20 stack, each player will be risking only $20; the excess $80 will be returned uncontested to the bettor. For this reason, even deep stacks need to be aware of short stack concepts when playing at a table with many shorter stacks.

Multiway effective stacks
The concept gets a bit complex when more than two players are involved in a hand. Suppose that Player A again bets all-in for $100, but is called by Player B with $50 and Player C with $20. Now the effective stack size is $20 among A, B, and C, but $50 between A and B.