The set of points in the plane where a function [math]f\left(x,y\right)[/math] has a constant value [math]f\left(x,y\right)=c[/math] is called a [i]level curve[/i] of [math]f[/math]. The set of all points [math]\left(x,y,f\left(x,y\right)\right)[/math] in space, for [math]\left(x,y\right)[/math] in the domain of [math]f[/math], is called the [i]graph[/i] of [math]f[/math]. The graph of [math]f[/math] is also called the [i]surface[/i] [math]z=f\left(x,y\right)[/math].

[i]Developed for use with Thomas' Calculus, published by Pearson.[/i]