Each input has exactly one output
No x-value is paired with more than one y-value
A relation is a function if every element in the domain maps to only one element in the range
If a graph passes the vertical line test, it is a function
If any vertical line intersects the graph more than once, it is not a function
In a table, repeated x-values must have the same y-value to be a function
In ordered pairs, the first coordinate cannot repeat with different second coordinates
In a mapping diagram, each input must point to only one output