Check whether each input (x) is paired with exactly one output (y)
If any (x) has more than one (y), it is not a function
If every (x) has only one (y), it is a function
For ordered pairs, look for repeated (x)-values with different (y)-values
For graphs, use the vertical line test
If any vertical line intersects the graph more than once, it is not a function
If every vertical line intersects the graph at most once, it is a function
For equations, solve for (y) if possible and check whether one (x) gives one (y)
If the equation produces multiple (y)-values for the same (x), it is not a function
For tables, verify that no input value is matched with more than one output value
For mappings, make sure each domain value points to only one range value