A transmission media is a material substance (solid, liquid or gas) which can propagate energy waves. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media for sound.
The absence of a material medium (the vacuum of empty space) can also be thought of as a transmission medium for electromagnetic waves such as light and radio waves. While material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media through which they pass, for instance by absorption or by reflection or refraction at the interfaces between media.
The term transmission medium can also refer to the technical device which employs the material substance to transmit or guide the waves. Thus an optical fiber or a copper cable can be referred to as a transmission medium.
A transmission medium can be classified as a:
Linear medium, if different waves at any particular point in the medium can be superposed;
Bounded medium, if it is finite in extent, otherwise unbounded medium;
Uniform medium or homogeneous medium, if its physical properties are unchanged at different points;
Isotropic medium, if its physical properties are the same in different directions.