In television, film, stage, or photographic lighting, a fill light (often simply fill) may be used to reduce the contrast of a scene to match the dynamic range of the recording media and record the same amount of detail typically seen by eye in average lighting and considered normal. From that baseline of normality, using more or less fill will make shadows seem lighter or darker than normal, which will cause the viewer to react differently, by inferring both environmental and mood clues from the tone of the shadows.
Natural skylight fill is omnidirectional and diffuse, with lower rate of inverse-square fall-off than artificial sources. A common artificial lighting strategy that creates an overall appearance similar to natural fill places the fill light on the lens axis so that it will appear to cast few if any shadows from the point of view of the camera, which allows the key light that overlaps it to create the illusion of 3D in a 2D photo with the same single-source patterns typically seen with natural lighting where the sun acts as key light and the skylight as the fill. The use of centered near-axis "neutral" fill also prevents dark, unfilled voids in the lighting pattern, which can occur on faces if cheeks or brows block the fill source.
