The types of transmission media include solids, liquids, gases and plasma; what all transmission media have in common is that they have the ability to propagate energy waves. Sound waves, for example, can pass through solids, liquids and air, and the effects of these different transmission media cause the sound to come through with various alterations.
One can consider how voices sound when they are across a room, on the other side of a wall or when the listener is underwater. Even when the original sound remains identical, the perception of the listener is going to be significantly different because of the effects that the specific transmission medium has on the sound waves.
When no material medium exists, the vacuum that exists often serves as a transmission medium for radio and light waves. These waves do not require a material substance to propagate, but when they do go through material transmission media, they undergo absorption, refraction or reflection, and the signal changes as a result.
Another definition for the term "transmission medium" refers to the actual device that puts the material medium to work for it. Examples include copper cables or optical fibers, and they facilitate the propagation of waves, whether electromagnetic, radio or sound.