In art, a medium is the material that artists use to create their art. It's that simple. Whatever a piece of art is made out of is its medium. The plural of medium is media.
The technique that was then being used in order to portray king Akhenaten, the 3 daughters and his wife Nefertiti was sunken-relief carving. This is a kind of carving that does not go on the flat surface and this is called cavo-relievo.