Transmission medium
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.
Source: from Federal Standard 1037C