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 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.
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.
Abdullah bin Abdulaziz Al Saud (In June 1982, when Fahd bin Abdulaziz became King, Prince Abdullah became Crown Prince the same day. He also maintained his position as head of the National Guard).
Sultan bin Abdulaziz, Sultan bin Abdulaziz Al Saud (30 December 1930 – 22 October 2011), called Sultan al-Khair (Arabic: سلطان الخير, Sultan of good) was the Crown Prince of Saudi Arabia, from 2005 to 2011.
Persia (Iran), Pahlavi 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'
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"): Japanese Kōtaishi, Korean Hwangtaeja (황태자), Vietnamese Hoàng Thái Tử -- if a son of the reigning emperor. In case the heir is a grandson, the title Huang Taisun (皇太孫), Kōtaison, Hwangtaeson (황태손), and Hoàng Thái Tôn, literally "Great Imperial Grandson", are used
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).