A function is a type of relation where each input has only one output


X    Y

3     6

4      8

2      9

This is a function because every input has only one output

X      Y

2      7

4        9

3       5

2       2

This is not a function because the input 2 has the output of 2 and 7

Another way of finding functions is the vertical line test if there is more than one point on the line it is not a function.