In a relationship between two variables x and y, the data set is a function, if every element of the domain corresponds to exactly one element of the range
that means
one element of x corresponds to exactly one element of y
In any function, there is an input value (independent variable or x variable) and there is an output value (dependent variable or y variable)
What makes a function a function? ------> one element of the input (variable x) corresponds to exactly one element of the output (variable y)