YourDictionary.com and Wiktionary.com define a "sugar mama" as a woman who spends money on a man, usually younger than her, in exchange for companionship or sexual favors. Usually the woman buys the man gifts or helps to pay his bills.
"Sugar mama," alternatively spelled "sugar momma," is a twist on the term sugar daddy. A "sugar daddy" is a man who is typically, but not always, older who buys gifts for a typically younger female in exchange for a sexual relationship, companionship or a mixture of the two. These can be either short- or long-term relationships.
Websites such as SeekingArrangement.com help good-looking young men, or "sugar babies," find sugar mamas looking for this type of relationship.