In politics and law, mediatisation ( /miːdiətaɪˈzeɪʃən/) 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.
Between 1803 and 1806, the vast majority of the states of the Holy Roman Empire were mediatised. These states lost their imperial immediacy (Reichsunmittelbarkeit) and became part of other states. The number of states was reduced from about three hundred to about thirty. Mediatisation went along with secularisation: the abolition of most of the ecclesiastical states.
The legal basis for mediatisation was the Reichsdeputationshauptschluss of 1803, which had become necessary under pressure from France. The Treaty of the Confederation of the Rhine of 1806 continued the process of mediatisation. The constitution of the German Confederation of 1815 confirmed the mediatisation, but preserved certain rights for the mediatised princes within their former realms, now ranked as state countries, such as first instance jurisdiction, supervision of religion and foundations.