Fill flash
From The Glossary of Digital Photography
Also called flash fill. A flash unit used with ambient light to create the exposure. The fill flash is used to add a bit of light when the subject is backlit. It is also used in outdoor portraits in the shade to add some contrast and a catch light to the eyes. It is used in sunlight to reduce the contrast by filling in (and thus the name) the shadows,
for example, on a subject's face.
