A common guideline to define a short stack is having around 20 big blinds or fewer.
It's a player who has a smaller number of chips compared to the average or maximum stack size at the table. The term can vary based on the context of the game and the blinds or stakes being played. Generally, a short-stack has a limited number of chips relative to the current blinds or the typical stack sizes of other players at the table. Short-stacked players often face strategic challenges due to their reduced chip count, needing to carefully choose when to enter hands and often facing difficult decisions about betting, calling, or folding with fewer chips available.
A common guideline to define a short stack is having around 20 big blinds or fewer.