Reflector
A panel that bounces existing light back into the shadows, adding fill without a second light source.
Measured in
stops
In depth
White gives the softest, most neutral bounce; silver is brighter and more specular; gold warms the light and is easily overdone; black is not a reflector at all but a negative fill that removes light. The key limitation is that a reflector can only return light it receives, so it must be placed where the existing light actually falls on it.
Examples in practice
A white reflector held waist-high in front of a backlit subject lifts the face by about a stop and a half.