Croatia proper () is one of the four historical regions of the Republic of Croatia, together with Dalmatia, Slavonia, and Istria. It is located between Slavonia in the east, the Adriatic Sea in the west, and Dalmatia to the south. The region is not officially defined, and its borders and extent are described differently by various sources. In contemporary geography, the terms Central Croatia () and Mountainous Croatia () are used to describe most of this area, the former referring to the northeastern part and the latter to the southwestern part; the far western part is known as the Croatian Littoral. Croatia proper is the most significant economic area of the country, contributing well over 50% of Croatia's gross domestic product. The capital of the Republic of Croatia, Zagreb, is the largest city and most important economic centre in Croatia proper.