optical thickness, <of the atmosphere>
optical depth, <of the atmosphere>
Note 1 to entry: See also "spectral optical thickness".
Note 2 to entry: The optical thickness has unit one.
Note 3 to entry: This entry was numbered 845-09-86 in IEC 60050-845:1987.
Note 4 to entry: This entry was numbered 17-850 and 17-844 in CIE S 017:2011.