Final answer:
In order for a relation to be a function, every input must have only one output.
Step-by-step explanation:
In order for a relation to be a function, every input must have only one output.
This means that for each x-value, there can only be one corresponding y-value.
For example, if we have a relation where (1, 2) and (1, 3) are both points, this would not be a function because the input value of 1 has two different output values.