Optical depth is a measure of how much light is absorbed in traveling through a medium, such as the atmosphere of a star, from the source of light to a given point. A completely transparent medium has an optical depth of zero. Optical depth depends on the frequency of radiation, as well the type of medium. For example, blue light is strongly affected by interstellar dust, so dust clouds have a high blue-light optical depth; on the other hand radio waves are unaffected by dust so the radio optical depth of dust clouds is zero.

Optical depth is much more significant to Earth's energy budget than the physical thickness of cloud or aerosol layers.