Real union is a union of two or more states, which share some state institutions as in contrast to personal unions; however they are not as unified as states in a political union. It is a development from personal union and was usually limited to monarchies.
Unlike personal unions, real unions almost exclusively led to a reduction of sovereignty for the politically weaker constituent. That was the case with Lithuania and Scotland which came under the influence of stronger neighbors, Poland and England respectively, with whom each of them had shared a personal union previously. Sometimes, however, a real union came about after a period of political union. The most notable example of such a move is the Kingdom of Hungary (Lands of the Crown of Saint Stephen), which achieved equal status to Austria (which exercised control over the "Cisleithanian" crown lands) in Austria-Hungary following the Austro-Hungarian Compromise of 1867.
Historical examples: