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 mak...
A light which makes areas in shadow more visible.
Positioned by the camera, a fill light is usually soft and illuminates those areas which are missed by other lighting devices. They reduce and eliminate shadows without producing shadows of their own and should not have a direct affect on the lighting exposure of the scene
Diffuse lighting used to complement Key Lighting. 
Light used to illuminate shadowy areas.
In the three-point lighting system, a source of illumination used to fill the shadows created by the
a light used to eliminate or soften shadows caused by the main source of illumination.