A crown prince or crown princess is the heir apparent to the throne in a royal or imperial monarchy. The wife of a crown prince is also titled crown princess.
The term is now borne as a title mainly in Asia, Scandinavia, and the Middle East; but it may also be used to refer to the person or position of the heir apparent in other kingdoms. However, heirs apparent to non-imperial and non-royal monarchies (i.e., wherein the hereditary sovereign holds a title below that of king/queen, e.g., grand duke or prince), crown prince is not used as a title, although it is sometimes used as a synonym for heir apparent.
In Europe, where primogeniture governs succession to all monarchies except those of the Papacy and Andorra, the eldest son (Spain) or eldest child (Belgium, Denmark, the Netherlands, Norway, Sweden, and the United Kingdom) of the current monarch fills the role of crown prince or princess, depending upon whether females of the dynasty enjoy personal succession rights. The eldest living child of a monarch is sometimes not the heir apparent or crown prince, because that position can be held by a descendant of a deceased older child who, by "right of representation", inherits the same place in the line of succession that would be held by the ancestor if he or she were still living (e.g., Carl Gustaf, Duke of Jämtland, was Crown Prince of Sweden from 1950 to 1973, as the senior grandson by male primogeniture of King Gustaf VI Adolf of Sweden, although the former Prince Sigvard, Duke of Uppland, was Gustaf VI Adolf's eldest living son, and Prince Bertil, Duke of Halland, his eldest living dynastic son during those years).