A Crown Prince or Crown Princess is the heir or heiress apparent to the throne in a royal or imperial monarchy. The wife of a Crown Prince is also titled Crown Princess.
In Europe, lineal succession conventions (see primogeniture) usually dictate that the eldest child (as in Sweden, Belgium, Norway, Denmark and the Netherlands) or the eldest son of the current monarch (Spain, United Kingdom, etc) fills this role, but in Arab monarchies, for example, succession rules may differ and a Crown Prince may gain the title on perceived merit, or because someone is not seen as a threat to the reign of the current monarch; in such cases a person granted the title may also lose it, with it being granted to another member of the Royal Family.
It should however be noted that, although it is often used as a generic term for heir apparent, it is often not an official title in the European monarchies. Currently, only the heirs apparent to the Scandinavian monarchies officially bear the title, while the heir presumptive normally would be titled Hereditary Prince.
Duke of Barcelos (Kingdom of Portugal) - this title was subsidiary to the title of Duke of Braganza, and was used by the House of Braganza heir during his father's life. When the Braganzas ascended to the Portuguese throne, the title of Duke of Barcelos was among those used by the Portuguese crown prince.
Many customarily (often not de jure) assign a primogeniture or award a hollow territorial title of princely rank; while often perceived as a crown princely title, these are not technically so, generally requiring a specific decision from the Sovereign, which may be withheld.
Current and past titles in this category include:
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.
Duke of Brabant (kingdom of Belgium) is not a true primogeniture: the title is not reserved, so it may still be occupied causing the Heir to be given another title, as present king Albert II remained Prince of Liège after his childless brother ascended the throne;
In Persia, under the Qajar dynasty, the full style was Vala Hazrat-i-Humayun Vali Ahad, Shahzada (given name) Mirza, i.e His August Imperial Highness the Heir Apparent, Prince ...;
the above component vali ahad 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 and taken from the Mughal title 'Vali Ahad'
In the Hindu tradition (Indian subcontinent):
Yuvaraja was part of the fullin many princely states of India, e.g.
in Kashmir, the Heir Apparent was styled Maharaj Kumar Shri Yuvaraj (personal name) Singhji Bahadur
Hoàng Thái Tử (太子) (Imperial Vietnam until 1945) was the term used by past dynasties for their Crown princes. Normally, a crown prince was also bestowed with other titles depending on the grace of the ruler. Hoàng Thái Tử, if the Emperor's son; Hoàng Thái Tôn (皇太孫) if a grandson