Original: Absorbance is defined as , with  as absorbance (Beer-Lambert law) and indicates degree of transparency (, the ratio of light intensity exiting the sample to that of the incident light) to a given wavelength of light.
Simplified: Absorbance is defined as , with  as absorbance (Beer-Lambert law). It is a measure of a given substance's transparency (, the ratio of light intensity passing through sample to that of the incident light) to a given wavelength of light.