http://digitalphotographyblogs.com/2006/02/18/what-is-rembrandt-light/

Rembrandt Light is a lighting effect that was used beautifully in many Rembrandt paintings (you know, the Dutch master painter). The effect is to light a person so only ¼ of their face is in shadow and there is a triangle of light under one eye. It is a universally flattering, yet dramatic light on faces. It allows for most of the face to be illuminated but with some interesting shadow detail. This is a Rembrandt self portrait done in 1629. You can see how the light looks like late afternoon sun coming through a window. The color is very warm and there is a little triangle of light under his left eye