**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

#f01

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.