I've seen it many times on this forum. Googled but couldn't find anything relevant 
Notifications
Clear all
Questions and Answers
6
Posts
5
Users
0
Reactions
1,300
Views
5 replies
It's short for buy in, the amount you choose to sit at a table with. For instance, when you have $50 in your account and you play 5nl, you have 10BI (buy in) for that limit. All given that you buy in for 100 big blinds, and not more or less 
As NIVEKii and Vildcz wrote already BI stands for buyin if it is poker related. 
If you have further questions, please do not hesitate to ask.
Good luck on the tables,
Hannes