A partial shadow between regions of complete shadow and complete illumination. From the word umbra, which means shadow. The penumbra on the cyan sphere you see here is located in two areas: between the brightest and the darkest areas, and also to the lower left of the darkest shadow. This latter area is the least obvious aspect to the shadowing on this sphere, because its presence is counter-intuitive, but when rendered, adds to the illusion of the sphere's three-dimensionality.(pr. pə-NUM-brə)