Transmission mediumTransmission medium
: Any material substance, such as fiber-optic cable
, twisted-wire pair, coaxial cable
, water, and air, that can be used for the propagation
of signals, usually in the form of modulated radio
, or acoustic waves, from one point to another.
Note: By extension, free space can also be considered a transmission medium for electromagnetic waves, although it is not a material medium.
Source: from Federal Standard 1037C