1In Germany under the Holy Roman Empire: to reduce (a prince or state) from the position of an immediate vassal of the Empire to that of a mediate vassal (now historical). Hence: to annex (a state) leaving the former government or ruler a title and (usually) some authority; also in extended use.
2More generally: to reduce (something) in power or effect by interposing a mediating agent. In recent use also: to render subject to interpretation or exploitation by the mass media; (of the mass media) to control or otherwise affect (an event, etc.).
Early 19th century; earliest use found in Benjamin Disraeli (1804–1881), prime minister and novelist. Either from French médiatiser from médiat + -iser, or from German mediatisieren from French médiat + German -isieren.