Wikipedia
In politics and law, mediatisation is the loss of immediacy. Immediacy is the status of persons not subject to local lords, but only to a higher authority directly, such as the Holy Roman Emperor. In a feudal context, it is the introduction of an intervening level of authority between a lord and his vassal so that the former is no longer the immediate lord of the latter, but rather his lordship is mediated by another. Although the process had been going on since the Middle Ages, the term "mediatisation" was originally applied to the reorganisation of the German states during the early 19th century. In this case, many states that were immediate vassals of the Holy Roman Emperor became instead vassals of other immediate states so that the total number of states immediately subject to the emperor decreased.