The percent of radiation returning from a surface compared to that which strikes it.
WRCC Climate and Weather Terms Glossary

The albedo is a measure of the reflective quality of a surface. Albedo is the ratio of the amount of light striking a surface to the amount reflected from the surface and is typically given as a percent. A surface having an albedo of 50% would reflect half of the light reaching it, whereas an albedo of 0% would indicate that no sunlight is reflected. Dry sand reflects approximately 35% of the sunlight that strikes its surface. Fresh snow reflects approximately 85%.
Education Glossary of Atmospheric Sciences Data Center

The ratio of the outgoing solar radiation reflected by an object to the incoming solar radiation incident upon it.
NASA Earth Observatory

Albedo - Refers to reflection power or the portion of solar radiation that reaches a body (earth) surface and is immediately reflected back into the atmosphere. The uncertainty in the reflective power of clouds unfortunately increases the complexity of determining clouds' effects on modeling global warming.
[New Scientist; v141; 16; 1994.]
[Sky Telescope; v85; 84; 1993.] 
Atmospheric Chemistry Glossary

АЛЬБЕДО - (от позднелат. albedo - белизна) - величина, характеризующая способность поверхности отражать падающий на нее поток электромагнитного излучения или частиц. Альбедо равно отношению отраженного потока к падающему. В астрономии важная характеристика планет и др. тел Солнечной системы.