1.someone that you like/love; girlfriend/boyfriend
2.someone of the opposite sex that has the potential to be hubby/wifey material; not quite boyfriend/girlfriend yet
3.could also be a friend or particular person you like
1. "I'm on the phone with my boo!!"
2. "Girl I got me a new boo!!"
3. Person 1: "Hey girl!"
Person 2: "Hey boo!"