Big Apple is a moniker or an alias for New York City. The Big Apple was first popularized as a reference to New York City as horse racing city in the 1920’s by writer John J. Fitz Gerald.

Later, between 1940 to 1950, came a popular song and dance titled “The Big Apple.” This time, the context of the song was no longer pointing to New York City as a horse racing city.

In the 1960’s the New York Convention and Visitor’s Bureau, began promoting “The Big Apple” as New York City’s moniker.

Finally, in 1997, Mayor Rudolph W. Giuliani signed a legislation designating the southwest corner of West 54th Street and Broadway, the corner where John J. Fitz Gerald resided from 1934 to 1963 as “the Big Apple” corner.(Source: Wikipedia, the Free Encyclopedia)