We can use level sets to visualize functions with multiple inputs and one output. A level set is a subset of the domain given by the implicit equations .

Level sets are often denoted as even when the function is not invertible.

In 2D, level sets look like curves on a plane. Those curves are often called contour line or isoline. elliptic_paraboloid_level_curves.png

In 3D, level sets look liker surfaces in the space. Those surfaces are often called isosurface.