A term for a relationship with a woman that must be sexual, but explicitly cannot be a dating relationship. Implies a lack of any commitment, but further implies that it is the most significant relationship said person has at the given time. The "main bitch" has the ability to get closer than other girls to her man emotionally, but probably doesn't want to if she knows that she is his "main bitch". With that being said, this is the only sexual relationship the man has at the time that even remotely has a chance of working out in the long run, though this chance is still infintesimally small.
Yo girls, my main bitch is calling, I gotta split out on you ladies. It was good getting to meet you though, we can hook up tomorrow. Peace