1. In radio transmission and reception, the area, zone, or volume of space (a) that is otherwise within range of a given transmitter, such as a radio, television, or radar transmitter, and (b) in which a signal is not detectable by a given receiver, such as a receiver behind a mountain or at the bottom of a mine shaft. 2. In fiber optic transmission systems, areas, regions, or spaces where it is not practical to enter, penetrate, or cross with a fiber optic cable. See also fiber optic transmission system,  radar,  radio,  receiver,  reception,  television,  transmission,  transmitter.