Why is New York called the Big Apple?

Hi! Thanks for the question.

The answers to this question are varied and at times conflicting ones.
I will direct you to different resources that attempt to answer the
question. Some say it is because of a jazz, others say it came about
because of horse racing, etc.

"Why Is New York City Called 'The Big Apple'?"
http://salwen.com/apple.html

"How did New York City come to be known as "The Big Apple"?"
http://www.nypl.org/research/chss/faq.html#apple

"Why is New York City called the 'Big Apple'?"
http://gonyc.about.com/cs/atozinde1/a/bigapple.htm

"Why is New York City called the Big Apple?"
http://www.infoplease.com/askeds/2-4-00askeds.html

"Why is New York called 'The Big Apple'?"
http://www.wsu.edu/DrUniverse/ny.html

Search terms used:  
why is "New York" called "Big Apple"

I hope these links would help you in your research. Before rating this
answer, please ask for a clarification if you have a question or if
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Easterangel-ga                              
Google Answers Researcher

Very Complete, several references

Thanks for the kind words, the 5 stars and for the tip! :)