1. Applied to an electromagnetic wave, often a radio wave, a drop in its power or field strength below a specified level. 2. Weakening of a received radio or radar signal caused by changes in the propagation medium. The term was originally applied to long-period changes in signal strength over bistatic radar links caused by interference between the signal reflected by the ionosphere and the direct, unreflected signal or ground wave. It now refers to interference phenomena caused by any type of multipath transmission either of tropospheric or ionospheric origin.