What is Absorbance?

ANSWER: 

(Absorptivity, Absorption Coefficient, Absorption Power) Absorbance is the log (base 10) of 1/Reflectance or Transmittance, denoted log(1/R) or log(1/T) respectively. Reflectance is the energy radiated from the sample divided by the radiant energy incident on the sample.

also see: Conservation of Energy