What Is A Medium?

The word medium can be used in a number of ways. But in all contexts, it is a mode of two ends to communicate. For example, if heat waves are conducted through the air then air is the medium. You cannot produce any sound in a vacuum because a medium (air) is missing for the sound waves to travel through. An optical medium, similarly, is a material through with electromagnetic waves can be propagated.
Similarly, the language in which a subject is taught at college is the medium of instruction your teacher is using. In business, medium can be referred to as the manner of payment. For example, the most common medium of payment is paper money. Gold is an international exchange medium.
A medium, in this context, is a person who can communicate with the spirits. Thus, that person is a means to communicate between the physical and spiritual worlds.
So, as you can see, the word medium may be used in a number of different ways but it essentially means that it is a means which takes one thing (a wave or a message) from one side of the material (or chasm) to the other.
The word medium also has another usage. It can be used to say that something is average, e.g. She is of a medium height. Medium is also used as a term in cooking which means not too tender or too well done.
That is about it!