(technically known as the relative aerosol optical depth) usually considered to be synonymous with the airmass, is the approximate number of aerosols in a path through the atmosphere relative to the standard number of aerosols in a vertical path through a clean, dry atmosphere at sea level.
A measure of the cumulative attenuation of a beam of radiation as a result of its travel through the atmosphere.
Optical depth is a measure of the transparency of a ring system. When a ring is "optically thick" (i.e., the optical depth is large), the ring is nearly opaque and very little light passes through. When a ring is "optically thin" (i.e., the optical depth is small), very little material is present and most of the light passes through.
Optical depth is a measure of transparency, and is defined as the fraction of radiation (or light) that is scattered or absorbed on a path. One way of visualizing optical depth is to think of a fog. The fog between you and an object that is immediately in front of you has an optical depth of zero.