Definition of shortstacker
A shortstacker is a player who voluntarily buys in cash game always for 20 big blinds. Playing shortstack means you have less than 100 big blinds. 100 big blinds is the reference. If you play 100 big blinds, you will be said to have a full stack. If you have 250 big blinds you will be said to be deepstack and if you always have in cash games around 20 big blinds, you will be said to be a shortstacker.
Shortstacker in cash game vs shortstack in tournaments
A shortstacker is not a shortstack in tournament. If you always buy for 20 big blinds in cash games, you are surely a shortstacker, i.e. a player who has perfected his shortstack game strategy. This shortstacker player has decided to be a shortstack, he knows perfectly well what he is doing. When you have 20 or 10 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", that is to say to make an aggressive EV+ game to, let's hope, double or triple our mat. The shortstacker in cash game and the shortstack in tournament don't have the same motives.