TRANSMISSION MEDIUM

Any material substance, such as fiber-optic cable , twisted-wire pair, coaxial cable , dielectric -slab waveguide , water, and air, that can be used for the propagation of signals, usually in the form of modulated radio , light , 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.