The National Football League (NFL) is a professional American football league, but it is often referred to by various other synonyms. Some commonly used synonyms for the NFL include 'American football', 'pro football', 'gridiron football', and 'NFL football'. Furthermore, some people use the term 'pigskin' to refer to the sport, as the football is made of pigskin leather. The NFL is also often called simply 'football' in the United States, as it is the most popular football league in the country. Other international football leagues, such as the English Premier League, may use 'American football' to differentiate between the two sports.