Practical Light
A light source that appears within the frame as part of the scene, such as a lamp, candle or neon sign.
In depth
Practicals do two jobs at once: they contribute illumination and they explain it, telling the viewer where the light is supposed to be coming from. They are usually too weak to light a subject properly, so the working method is to place a hidden source that mimics the practical's direction and colour while the practical itself simply reads in shot.
Examples in practice
A table lamp in the corner of a frame motivates a warm-gelled light just out of shot doing the actual illuminating.