Fading

http://en.wikipedia.org/wiki/Fading

In wireless communications, fading is deviation of the attenuationthat a signal experiences over certain propagation media. The fadingmay vary with time, geographical position or radio frequency, and isoften modelled as a random process. A fading channel is a communication channel that experiences fading. In wireless systems, fading may either be due to multipath propagation, referred to as multipath induced fading, or due to shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading.