Preprocessor

   >>> The preprocessor looks for preprocessor construction, each of which begins with a pound symbol (#).


   >>> Using #define 


   >>> The #define command defines a string substitution. If you write #define BIG 512.

  >>> You have instructed the pre-compiler to substitute the string 512 wherever it sees the string BIG. This is not a string in the C++ sense. The characters 512 are substitute in your source code wherever the token BIG is seen. A token is a string of characters that can be used wherever a string or constants or other set of letters might be used.

   >>> With the use of above code you are going to define a Constant Value.