Corpus separatum, a Latin term meaning "separated body", refers to the status of the City of Fiume while given a special legal and political status different from its environment under the rule of the Kingdom of Hungary. Formally known as City of Fiume and its District, it was instituted by Empress Maria Theresa in 1779, determining the semi-autonomous status of Fiume within the Habsburg monarchy until the fall of the Austro-Hungarian Empire in 1918.

Rijeka, also known as Fiume, is the principal seaport and the third-largest city in Croatia. It is located in Primorje-Gorski Kotar County on Kvarner Bay, an inlet of the Adriatic Sea and in 2021 had a population of 108,622 inhabitants. Historically, because of its strategic position and its excellent deep-water port, the city was fiercely contested, especially between the Holy Roman Empire, Italy and Croatia, changing rulers and demographics many times over centuries. According to the 2011 census data, the majority of its citizens are Croats, along with small numbers of Serbs, Bosniaks and Italians.