Albedo is the ratio of the outgoing solar radiation reflected by an object to the incoming solar radiation incident upon it.
Albedo is the reflection coefficent of an object to electromagnetic radiation (usually visible light). The word comes from the Latin word albus, meaning "white."
Albedo is not expressed in any units but on a scale from 0 to 1. A pure white object would be 1 while completely black object (e.g. black hole) would have a value of zero. It is an important concept in climate change discussions where the albedo of the earth affects the amount of light and heat reflected back in to space.
The moon has an albedo of 0.39.