Definitions:
an agency or means of doing something

the intervening substance through which sensory impressions are conveyed or physical forces are transmitted

a particular form of storage material for computer files, such as magnetic tape or discs.

the material or form used by an artist, composer, or writer

a person claiming to be in contact with the spirits of the dead and to communicate between the dead and the living.

the middle quality or state between two extremes; a reasonable balance

about halfway between two extremes of size or another quality; average