breed of dog, originally called Staffordshire terrier when registered with American Kennel Club in 1936, that was developed in the United States and based on the smaller British Staffordshire bull terrier. Its ancestry includes the breeds used for bullbaiting and dog fighting. Authorities differ on whether the American Staffordshire terrier and the American pit bull terrier are the same breed, but they agree that properly bred and socialized dogs do not display innate aggression against humans.