A Function is a very common word we use in mathematics. However very few know the exact meaning of a function. A function has one input and one output. An input is X and output is Y. If you give X to function you will get Y. So there are some ways to recognize functions. For each X value has to be only one Y value. When you graph a relation if you find a vertical line that has two or more intercepts with your relation, it is not a function.