Can anyone explain to me how can I tell if a relation is a function.

An example can be a parabola or anything related to my question.

A function is a relation such that for each first element (x - value) there exists one and only one second element.

Another way to say this is that none of the ordered pairs have a repetitive x-value.

example: C = { (2,5), (6,1), (8,2), (9,5), (4,10) }

Set C is a function because each first element is only listed once.