The American Quarter Horse is an American breed of horse that excels at sprinting short distances. Its name came from its ability to outdistance other horse breeds in races of a quarter mile or less; some have been clocked at speeds up to 55 mph (88.5 km/h). The American Quarter Horse is the most popular breed in the United States today, and the American Quarter Horse Association is the largest breed registry in the world, with almost 3 million American Quarter Horses currently registered.