Mediatization, defined broadly, is the annexation of one sovereign monarchy by another sovereign monarchy in such a way that the ruler of the smaller state keeps their noble title (and sometimes, a measure of power). Thus, for example, when a sovereign duchy is annexed to a larger principality, its reigning duke might find himself subordinated to a prince, but would nevertheless remain a duke (rather than be stripped of his title). Often, mediatized noble houses were officially ranked higher than other houses of nominally equal rank—a duke whose family had once ruled a sovereign duchy was given more prominence than a duke whose family had not.

The most notable example of mediatization came in the early 19th century, when many small German fiefdoms were incorporated into larger states. This process is sometimes known simply as "the Mediatization". There are a number of other countries in which mediatization has occurred, however, with Italy and Russia being notable examples.

See also: list of mediatized houses.