Definition of Function

Here are some definitions of FUNCTION:

(THESE ARE NOT MY WORDS...I GOT THESE DEFINITIONS FROM THE WEBSITES IN THE LINKS BELOW THE DEFINITIONS OR PICTURES)

• A rule for creating a set of new values from an existing set; for example, the function f(x) = 2x creates a set of even numbers (if x is a whole number).
  From : http://www.pcmag.com/encyclopedia_term/0,2542,t=mathematical+function&i=46629,00.asp
  
• A function associates to each element in the set X (an input) exactly one
  element in the set Y (the output). Different elements in X can have the same
  output, and not every element in Y has to be an output.



• 
  http://en.wikipedia.org/wiki/Function_(mathematics)