Final answer:
A relation is a function if each input has only one output.
Step-by-step explanation:
A relation is a function if each input has only one output. In other words, for every x-value, there can only be one corresponding y-value. This means that each x-value is unique in the relation.
For example, the relation {(1, 2), (3, 4), (5, 6)} is a function because each x-value (1, 3, 5) corresponds to only one y-value (2, 4, 6).