In mathematics, a function is a relation between a set of inputs and a set of permissible outputs. Specifically, each input into a function has exactly one corresponding and correct output.
One example of a function is the relation between x (any real number) and twice that number, or 2x. The function would then be written with the notations f(x) (read "f of x" or "function of x") and 2x (the function itself). It would be written as f(x)=2x. For the input 3, the output would be 6, which is 2 times 3. For the input -3, the output would be -6, and so forth.