Exposure compensation
From The Glossary of Digital Photography
Scenes photographed with very bright or very dark subjects, or regular subjects against a very bright or very dark background, will often need to have their exposure adjusted from the 'average' one calculated by an automatic camera. This is usually done by setting an exposure adjustment or compensation in one-third or one-half stop increments on the digital camera. These adjustments can be either positive or negative, and there is usually a possibility of having up to two or three stops in total. Some trial-and-error experimentation is often required to get the perfect exposure.
