What Is the Definition of Fill Light in Photography?

Fill light is form of supplementary light mainly used to lighten shadows in an image. Fill light is often used in portrait photography to create a contrast between the image subject and image background giving the scene a sense of depth despite the final product being 2 dimensional. In this situation the use of fill light also reduces the overall dynamic range of the scene allowing for easier selection of the exposure settings required to capture an image. When fill light is correctly applied it does not significantly impact the main light source of an image.