Pillow Shading refers to gradients of tints, tones and shades which hug outlines or contours instead of following any light and shadow logic by imagined light as it would illuminate volumes in three dimensional space.

Although with a straight frontal light source of an infinite amount of parallel rays going into the picture plane, objects might appear a tiny bit similar to as they appears when pillow shaded, pillow shading is the result of a flat, shape based, two dimensional thought process which considers neither perspective nor volumes, nor the nature of how light is manipulated as it hits surfaces.