A transmission medium is 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