The measure of concentration of material present; the negative log (base 10) of transmittance [–log 1/T] of product of extinction coefficient, path length, and concentration, written as...

see Optical...

In spectroscopy, the absorbance A is defined as A_\lambda = \log_{10}(I_0/I)\,, where I is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light...