A function is a rule that assigns each vale of the independent variable to exactly one vale of the dependent variable.
The missing word is the function.
Point to remember:
The definition says that two values of x(independent variable ) may correspond to one value of y(dependent variable) but the vice versa is not true( means two values of y(dependent variable ) does not correspond to one value of x(independent variable) ). This explanation is very powerful tool to check that whether a relation is a function or not.
