A term used to define an unmarried young woman (but can be a woman of any age) who has had a child. As mentioned before in another definition, most of the time it is used for when it was simply a sexual relationship, compared to ex-wife or girlfriend. Usually this has a negative connotation, a lot of baby mamas are seen as desperate, gold digging, emotionally starved, shady women who had a baby out of spite or to keep a man. Sometimes they may act like this because of missed child support payments, unfulfilled promises by the father, or convenient sex by the father. Either or both may exist in any situation.
Joe didn't have any relationship with that chick, she was the "other woman" who ended up being his baby mama.
Maybe Andy's baby mama is pissed because he said he was taking his sons to get a haircut but instead ended up playing dominoes with his boys.
Tiffany is a greedy baby mama; she is using her child support payments to buy her new boyfriend some speakers for his car.