The
Big Apple is a
colloquial name for
New York City. Occasionally shortened to '
The Apple.'
According to Barry Popik, an obsessive NYC parking ticket judge with an off=hours hobby of etymology, the term 'big apple' originated in popular usage with a writer for the now-defunct newspaper New York Morning Telegraph by the name of John J. Fitz Gerald. Mr. Fitz Gerald covered the horse racing circuit, and referred to New York City and its associated plethora of racing circuits as 'the big apple' collectively. Mr. Popik hunted through the microfilm of the Telegraph and according to his research (available on the Museum of the City of New York's web page) Mr. Fitz Gerald first heard the term used by some jockeys in New Orleans in January of 1920, where (apparently) the high concentration of races and purses around New York City had earned it the moniker 'the big apple' in horse racing parlance. He began to use the term to refer to New York City in his columns.
In 1971, New York City sponsored an advertising campaign to attract tourism. To counter the city's image as a dark, dangerous place - something that movies of the late 1960s and early 1970s certainly weren't helping - the ad campaign chose as its symbol a large, cheerily red apple, and the term 'The Big Apple.'
While there are other competing and coexisting explanations, these two are the most commonly found for the popularity of the term today.