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 generically 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 or (more recently) eldest child (Belgium, Denmark, the Netherlands, Norway, Spain,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).
In some monarchies, those of the Middle East for example, in which primogeniture is not the decisive factor in dynastic succession, a person may not possess the title or status of crown prince by right of birth, but may obtain (and lose) it as a result of an official designation made on some other legal or traditional basis, e.g., former Crown Prince Hassan bin Talal of Jordan.
Compare heir apparent and heir presumptive. In Scandinavian kingdoms, the heir presumptive to the crown may hold a different title than the heir apparent: Hereditary Prince (German: Erbprinz, French: prince héréditaire). It is also the title borne by the heir apparent of Liechtenstein, as well as the heir apparent or presumptive of Monaco. In Luxembourg, the heir apparent bears the title of Hereditary Grand Duke (German: Erbgroßherzog, Luxembourgish: Ierfgroussherzog); along with Hereditary Prince, it was also the title borne by the heirs apparent to the thrones of the grand duchies, sovereign duchies and principalities, and of mediatized princely families in the German monarchies abolished in 1918.
Christian/Western traditional titles
Many monarchies use or did use Substantive title for their heirs apparent, often of historical origin:
Prince of Orange (Netherlands), whether or not the equivalent title is held by the spouse of the titleholder is decided by the Dutch parliament (e.g., Queen Máxima of the Netherlands was never titled Princess of Orange by marriage for this reason)
Some monarchies have used (although not always de jure) a territorial title for heirs apparent which, though often perceived as a crown princely title, is not automatically hereditary. It generally requires a specific conferral by the sovereign, which may be withheld.
Current and past titles in this category include:
Caesar or Kaisar (Roman and early Byzantine Empires) in honor of Gaius Julius, distinguished from the senior Augustus
Symbasileus (late Byzantine Empire), lit. co-emperor but still distinguished from the senior who was addressed as Autocrator
Aetheling (Anglo-Saxon England) and edling (Welsh kingdoms), lit. of the royal family
King of the Romans (Holy Roman Empire) - an elective, rather than an inherited title, for the designated successor—usually the son, but sometimes the brother—of the Emperor
Persia (Iran), Pahlavi dynasty and Qajar dynasty, the full style was Vala Hazrat-i-Humayun Vali Ahd, Shahzada (given name), (in Persian: والاحضرت همایون ولایتعهد) i.e. His August Imperial Highness the Heir Apparent, Prince ...;
the above component vali ahd (or Velayat Ahd) meaning 'successor by virtue of a covenant' (or various forms and etymological derivations) was adopted by many oriental monarchies, even some non-Muslim, e.g. Walet as alternative title for the Nepali (Hindu) royal heir apparent; first used Crown Prince Trailokya in the middle of the nineteenth century, taken from the Mughal title 'Vali Ahd'
Hindu tradition (Indian subcontinent):
Yuvaraja was part of the full title in many princely states of India, e.g.
in Jammu & Kashmir, the heir apparent was styled Maharaj Kumar Shri Yuvaraj (personal name) Singhji Bahadur
Tika
Nepal, where the King was styled Maharajadhiraja:
the heir apparent was styled: Sri Sri Sri Sri SriYuvarajadhiraj ('Young King of Kings', i.e. Crown Prince) (personal name) Bir Bikram Shah Deva;
the eldest son of the heir apparent was styled: Sri Sri Sri Sri Sri Nava Yuvaraj ('Young Crown Prince') (personal name) Bir Bikram Shah Deva
East Asian traditions:
The cognates of Chinese Huang Taizi (皇太子, "Great Imperial Son") - if a son of the reigning emperor, and Huang Taisun (皇太孫, Great Imperial Grandson) - if a grandson of the emperor:
During the Joseon Dynasty in Korea, the crown prince was referred as Dong-gung(동궁, 東宮) due to the location of his residence from the main palace; or wangseja (王世子 왕세자). He was not necessarily the first-born son, wonja (元子 원자).
Southeast Asian traditions:
Siam Makutrajakuman (สยามมกุฎราชกุมาร) in Thailand since 1886.
Krom Phrarajawangboworn Sathanmongkol or Phra Maha Uparaja or commonly called Wang Na (or Front Palace) in Thailand prior to 1886.
Kanjeng Gusti Pangeran Adipati Anom in Yogyakarta sultanate and Surakarta, Indonesia.
Raja Muda or Tengku Mahkota in the Malay sultanates of Malaysia.