|3dRender.com : Glossary : Motivation|
Motivation - The perceived source of any illumination, shadow, or visual effect that appears in an image (when the term is used in reference to lighting). For example, we would say "The flickering blue light on the side of the character's face is motivated by the television set in the room." Or "The motivation for the light coming through the window is that a car is supposed to be passing by on the highway outside the hotel."
Source: Digital Lighting & Rendering by Jeremy Birn, definition and 3D images adapted from the book by permission. This page © 2002 by Jeremy Birn.