The term absorbance can refer to several scientific concepts. Absorbance can be used as a blanket term that measures any attenuation, or reduction, of transmitted radiant power. Attenuation can be caused by absorption, but also by reflection, scattering, and/or other physical processes. An example of this how TV and radio signals decrease in power as distance from the transmission tower increases. This is why distant TV and radio signals are faint and begin to have static.
For example, you are listening to your favorite radio station at the beginning of a road trip. As you travel farther your signal keeps getting weaker until it becomes nothing but static. This is the result of signal absorbance.
In chemistry, absorbance is a measure of the amount of light that is absorbed as it passes through a substance/sample. Typically wavelengths of the light are measured. In biological studies absorbance is used to measure bacterial growth in suspension cultures. Measurements are often referenced as 'Absorbance Units' or AUs.