Bred to fight in rings called “pits.”
The term 'pit bull' doesn't have a specific meaning. To some, it means any Staffordshire Bull Terrier, American Pit Bull Terrier, American Staffordshire Terrier, or Bull Terrier. To some, it means any dog with a blocky head and stocky body. To others, any descended from an American bulldog, a French Bulldog, or a Boston Terrier.
The name "pit bull" is derived from the fact most are descended from bulldogs, and the Staffordshire Bull Terrier, American Pit Bull Terrier were bred to fight in pits.
However, the American Staffordshire Terrier has only been a recognized breed since 1936, and was developed a half-century or more after the other dogs. It was bred as a family dog and was never bred as a fighting dog. I wouldn't hesitate to buy an American Staffordshire Terrier from a reputable breeder, although it's a 'pit bull.'