medium locus

a. substance through which a wave travels—air, water, ether; b. material an artist works in; c. one who channels the dead; d. means of communication; e. at neither extreme (as in size).

Getty AAT: The materials that are applied to a support in the making of a work of art, such as ink on paper or oil paint on canvas; often a particular technique or techniques are closely associated with the media. (AAT 300163343)