(1) The measure of transmittance of an optical medium for a given wavelength. The higher the OD the lower the transmittance, the higher protection factor by a filter (e.g. goggles, viewing windows, etc.).
(2) The measure of the amount of light absorbed by a suspension of bacterial cells or a solution of an organic molecule with the use of a colourimeter or spectrophotometer.
In microbiology, the values can be used to measure turbidity, which in turn is used to estimate the number of bacteria (or concentration of molecules like proteins) in a solution.
See also: absorption.