In spectroscopy, the absorbance A is defined as

A_λ = - log₁₀(I / I₀),

where I is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light intensity) and I₀ is the intensity of the light before it enters the sample (or incident light intensity). 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, in contrast to the transmittance I / I₀ of a sample, which varies exponentially with thickness and concentration. See the Beer-Lambert law for a more complete discussion.

Note that the term absorption refers to the physical process of absorbing light, while absorbance refers to the mathematical quantity. Also, absorbance does not always measure absortion: if a given sample is, for example, a dispersion, part of the incident light will be in fact scattered by the dispersed particles, and not really absorbed. Absorbance only contemplates the ratio of transmitted light over incident light, not the mechanism by which light intensity decreases. Despite of this, absorbance can still be used to determine concentrations (of particles) in such cases.

Outside the field of analytical chemistry, the absorbance is sometimes defined as the natural logarithm instead of the base-10 logarithm.

Any real measuring instrument has a limited range over which it can accurately measure absorbance. An instrument must be calibrated and checked against known standards if the readings are to be trusted. Many instruments will become non-linear (fail to follow the Beer-Lambert law) starting at approximately 2 AU (~1% Transmission). It is also difficult to accurately measure very small absorbances that are very close to zero absorbance. The theoretical best accuracy for most instruments is in the range near 1 AU. When possible, the path length or concentration should be adjusted to achieve readings near this range. It is extremely difficult to achieve an instrument range as large as 0-6 AU.

Optical density, or OD, is the absorbance per unit length, i.e., the absorbance divided by the thickness of the sample.