The 1969 New York Mets were not supposed to be as good as they were. They were an expansion team who had never finished better than 9th place out of 10 teams in the National League. 1969 was the first year the divisions were created and the Mets played in the National League East.
On September 24th 1969, the Miracle (or Amazin) Mets clinched the NL East! After starting the season 18-23, the Mets shocked everyone by winning ballgames! On August 14th the Mets were 10 GB of the Cubs in the divsion. From that point on they went 38-11 to win the division by 8 games earning them the names the Amazin Mets and the Miracle Mets.
Led by the arms of Tom Seaver and a Jerry Koosman the Mets completed one of the most incredible comebacks in baseball history but they did not stop there. The Mets would of course go on to beat the Orioles in the World Series that year 4-1.
The nickname for that team stuck and is now used as a nickname for the team every year!