Answer: function
Explanation:
Functions are relationships or rules that assign each value of the independent variable (Domain) exactly one value of the dependent variable (Range)
For example if we have the following function:

The independent variable is x.
Notice that when:

In this way the function assigns an output value to each input value.
Therefore the answer is:
A_function_ is a rule that assigns each value of the independent variable to exactly one value of the dependent variable.