1. The (dimensionless) line integral of the absorption coefficient, or of the scattering coefficient, or of the sum of the two, along any path in a scattering and absorbing medium. More often than not, the qualifiers “absorption,” “scattering,” and “total” are omitted (context sometimes being sufficient to determine which is meant). Optical thickness also depends on the wavelength of the radiation of interest. In a uniform medium, optical thickness has a simple physical interpretation as the length of a path in units of mean free path. Optical thickness and optical depth are used more or less synonymously; if there is a distinction between them it is that optical thickness is applied to an entire path through a medium. Normal optical thickness is optical thickness along a vertical path. Compare optical pathlength. 2. The degree to which a cloud modifies the light passing through it. Optical thickness depends on the physical constitution (crystals, drops, droplets), and the form; the overall effect depends on the scatter parameter and the phase function for the particles as well as their concentration and the vertical extent of the cloud.