Hey xotinalx.

For 2D function, the easiest way to know if something is a function is if you can draw a vertical line anywhere and it only crosses the function once. So as an example the relationship x^2 + y^2 = 1 is not a function unless you restrict it further but y = x^2 is.

Mathematically this means that if y = f(x) then it means that if you have two values where a = f(b) and c = f(d), then something is a function if and only if b = d implies that a = c (or in other words, I give you an x and you can only give me back 1 y).