A short stack refers to a stack which is relatively small compared to the other stacks at the table. The short stack refers to the smalled stack at a given table.

Size of a short stack

In a game like hold'em, anything less than 50BB is generally considered to be a short stack. Though one is better off considering the relative sizes of the stacks rather than compared to the blinds, or minimum betting. For even if you have 100BB, if every other player has 300-400BB you still have a short stack.

Strategy

Playing with a short stack requires a different type of strategy than playing with a larger stack size. This is required since your available options are somewhat to greatly limited by your stack stack size.

By no means complete, here is a list of some limitations due to a short stack:

• bluffing can be difficult as you may not have enough money to significantly alter pot odds
• you are particularly vulnerable to being put all in, thus being forced to call on a less than ideal hand, or being forced to fold more often than you wish
• other players tend to play more hands than they usually do as their implied pot odds will be very close to their current pot odds
• once your stack has shrunk enough you'll be forced to either fold or go all in with your hand

In contrast this loss of options is a distinct advantage in certain situations. In particular it lets you go all in without fear of losing much money. In addition the limitations of your actions is also a constraint on other players. They will be unable to use any complex bluffing, nor slow playing tend to get them any additional returns.