The Arizona Diamondbacks (colloquially known as the D-backs) are an American professional baseball team based in Phoenix, Arizona. The Diamondbacks compete in Major League Baseball (MLB) as a member club of the National League (NL) West division. The franchise was established as an expansion team and began play in 1998. The team plays its home games at Chase Field, formerly known as Bank One Ballpark. In its fourth season, Arizona won a World Series championship, defeating the New York Yankees in 2001 to become the fastest expansion team in major league history to win a championship.