Relation vs Function
The difference between a function and a relation is that for a function each domain value (x value) should be matched to one and only one range value (y value), while for a relation the domain value can be matched to more than one range value. This difference is shown using an arrow diagram below, for the relation:
y = 2 ± √ x, for x = 4, 9, 16
Note: The y values in the arrow diagram above, were obtained by substituting the respective x values in the equation, y = 2 ± √ x, for x.