Line art

Two-dimensional or three-dimensional objects are often represented through shade (darkness) or hue (color).

Line art may tend towards realism (as in much of Gustave Doré's work), or it may be a caricature, cartoon, ideograph, or glyph.

An important feature of a line is that it indicates the edge of a two-dimensional (flat) shape or a three-dimensional form.

A shape can be indicated by means of an outline, and a three-dimensional form can be indicated by contour lines.

[1] Before the development of photography and of halftones, line art was the standard format for illustrations to be used in print publications, using black ink on white paper.

Example of line art (published in The Survey , October 1917–March 1918).
Art Nouveau line art
Line art by Leonardo da Vinci