A General Function points from each member of "A" to a member of "B".
To be a function you never have one "A" pointing to more than one "B", so one-to-many is not OK in a function (as you would have something like "f(x) = 7 or 9")
Injective, Surjective and Bijective
What about a circle? Is that not a function? If not, why?