Question 480715
You could say,


"If any vertical line crosses the graph of a relation at at most one point, the relation is a function."


"If some vertical line crosses the graph of a relation at two or more points, the relation is not a function." (this statement is necessary since it is not logically equivalent to the first statement)


Alternatively, you can condense into one if-then statement:


"Iff any vertical line crosses the graph of a relation at at most one point, the relation is a function." (Iff means "if and only if").