the capacity of a substance to absorb radiation, expressed as the common logarithm of the reciprocal of the transmittance of the substance.

noun

physics a measure of the light-absorbing ability of an object, expressed as the logarithm to base 10 of the reciprocal of the internal transmittanceSee transmittance

n.

In spectrophotometry, equal to 2 minus the log of the percentage transmittance of light.extinction optical density