Definition of deepstack
If we break down the word, we have two words: deep and stack. When you are deepstack, it means that you have a deep stack, i.e. a large stack. Since cash games are usually played at 100 big blinds, we will take 100 as a reference. 20 big blinds or less, you are shortstacked. 50 big blinds, you have a medium stack. ?? 100 big blinds, you're a classic stack. ?? say (the number is arbitrary, but it gives you an idea) 175 big blinds and more, you're starting to go deepstack. In some online poker rooms, you can find cash game tables that offer 250 big blinds. Now you're really deepstacked!
What's the difference being deepstack?
The more chips you have, the harder it is to play and the more decisions you have to make. When you have a pair of preflop 9s with 5 big blinds, you push all in and you're set. But when you have the same pair of 9s with 250 big blinds, you can't just push all in with your hand. We'll have a lot more decisions to make. The good news is that the more you dominate the competition, you'll want to go all-in against them. Your advantage over the competition will be more obvious when everyone is deepstacked.
