The greatest distance in a given direction at which it is just possible to see and identify with the unaided eye
A measure of the extinction due to scattering of monochromatic radiation as it traverses a medium containing scattering particles.
The extinction coefficient for a particular substance is a measure of how well it scatters and absorbs electromagnetic radiation (EM waves). If the EM wave can pass through very easily, the material has a low extinction coefficient. Conversely, if the radiation hardly penetrates the material, but rather quickly becomes extinct within it, the extinction coefficient is high.