Definition of shortstack
Gambling shortstack means you have less than 100 big blinds. 100 big blinds is the reference. If you play shortstack0 big blinds, it will be said that you have a full stack. If you have 250 big blinds we'll say you're deepstack, and if you have around 20 big blinds, we'll say you're shortstack. There are two types of shortstack, the shortstack in cash game which is a voluntary shortstack and the shortstack in tournament which is an involuntary shortstack.
Shortstack in cash game vs shortstack in tournament
If you always buy for 20 big blinds in cash games, you may be a shortstack pro, i.e. a player who has perfected his shorthanded game strategy. This shortstack player has decided to be shortstack, he knows exactly what he is doing. When you have 20 or shortstack or 5 big blinds in tournaments, it's not by choice; the hands haven't always been the way we wanted them to be. In these moments, we will feel the urgency to make a "move", i.e. to make an aggressive EV+ game in order, hopefully, to double or triple our mat. The shortstack in cash game and in tournament will not be the same type of player.