Level surface
= Level surface
{title2=$f(x,y,z)=c$}
A <level set> of a real-valued function on three-dimensional space. Where the function is continuously differentiable and its <gradient> is nonzero, the <regular level set theorem> makes this a smooth surface with <normal vector> parallel to the gradient. The equation alone can also have singular points, or fail to describe a two-dimensional surface at all.