1historical In Germany under the Holy Roman Empire: the action of reducing a prince or state in position or power; the condition of being mediatized.
2More generally: reduction in power or effect by the interposition of a mediating agent, or as a function of a medium of transmission; the action or process of making something indirect in its application.
3Specifically. The action or process whereby the mass media comes to control or affect something.
Early 19th century; earliest use found in The Edinburgh Review.