This is one of the five basic lighting setups used in studio portrait photography; the others being Broad, Short, Split Portrait and Butterfly. There are two things that make up Rembrandt lighting: a light on one half the subject’s face and a triangle of light on the shadowed side of the face. Technically, the triangle shadow should be no wider than the eye and no longer than the nose. The thing that distinguishes Rembrandt lighting from simple short lighting is the triangle of light.
This method can be considered simply a variation of short lighting. When the lighting is such that the shadow of the nose reaches the shadow side of the face and forms a triangle on the short side of the face, it is referred to as Rembrandt lighting.