What is the definition of function?

The correct answer is: Every input has only ONE output

 * Function is generally represented as  Then, we can write the statement that f is a function from X to Y.

** Hence, A function is more formally defined given a set of inputs X(domains) and a set of possible outputs Y(codomain) as a set of ordered pairs (x, y). 
*** Hence, we can say that if the object x is in the set of inputs then a function f will map the object x to exactly one object f(x) in the set of possible outputs.  
*** Hence, we can say that for a function there exists a different output for any input.

Let us consider a function that defined as  then, the representation may exist as shown below.

Hence, Every object x that belongs to the set of inputs has exactly one object y that belongs to the set of outputs.
*** Hence, the technical definition of the function is a relation from a set of i9nputs to set of possible outputs where each input is related to exactly one output.