When trying to define 'medium' it should also be remembered that certain mediums were used to provide a communication channel for the transmission of teaching and guidance from spiritually-evolved individuals.

In these communications there was no intentional evidential mediumship and yet the medium acted as a go-between for discarnate to incarnate communication. In such cases perhaps the term 'channelling' might be more suitable?

Nowadays I increasingly use the term 'evidential medium' where communications are of a personal nature from an identified communicator.