What is the general name for a surface of form $z = f(x, y)$? Is there a generic, geometrical name for a "sheet" of data, where the data fits the form: 
$$z = f(x, y)$$
Note that $z$ might be an actual third dimension (such as elevation in terrain data) or any scalar measurement (such as surface temperature).
I know you could generically say "it is a surface"... but a sphere or a cube also are surfaces, and I want to exclude them (from the term I'm looking for) because they are not single-valued in $f(x, y)$.
Note I'm not trying to get too picky that the surface is continuous in $(x, y)$ or discrete.  The dataset might be infinite in range, or only exist for some constrained region in $(x, y)$.  And I'm not concerned if the surface is continuously differentiable.  I'm looking for the term that glosses over all those.
I realize this is might just be called "planar data".  And in some contexts (e.g. terrain maps in video games) I think this may be called "two-and-a-half dimensional data".
But is there a well-established term in geometry or topology for this?
Thanks!
UPDATE:
I finally stumbled across the term I was looking for: "scalar field".  Perhaps this term is more common in physics than math; since my application is in the physical sciences (terrain elevation and radio propagation measurements) I'm probably going to use that in my work.
I really appreciate all who replied.

Non-parametric or graph form are two commonly employed words.
