A planar image is one that renders shadows as a solid dark color, and lighter parts as a solid light color
or white, with no shading (1-10). A silhouette is similar but uses only the contour of the whole subject and disregards any light falling on it (11-20).
(Excerpt from Logo Theory: How Branding Design Really Works,
Section 29: Visual Technique #2: Planar or Silhouette)