1. A measure of the attenuation caused by absorption of energy per unit of distance that occurs in an electromagnetic wave of given wavelength propagating in a material medium of given refractive index. Note: The value of the absorption index K' is given by the relation
where K is the absorption coefficient, is the wavelength in vacuum, and n is the refractive index of the absorptive material medium. [After 2196] 2. The functional relationship between the Sun angle--at any latitude and local time--and the ionospheric absorption.