BK commonly stands for:
1. Burger King, a popular fast food restaurant chain.
2. Brooklyn, a borough in New York City.
3. Bankruptcy, a legal term referring to an person's or entity's inability to repay debts.
4. Back, often used in text messages or chats.
5. .bk, a file extension for backup files in computing.
It could also refer to other things depending on the context.
