Absorbance measurements are often carried out in analytical chemistry, since the absorbance of a sample is proportional to the thickness of the sample and the concentration of the absorbing species in the sample.
Although absorbance does not have true units, it is quite often reported in "Absorbance Units" or AU.
Word origin: L absorbére, equiv. to ab- + sorbére to suck in, swallow + -ance.
Related forms: absorbency (noun).