A Crown Prince or Crown Princess stands first in line to succeed to the throne in a royal or imperial monarchy. In Europe, lineal succession conventions (see for example Salic law) usually dictate that the eldest child (as in Sweden, Belgium, Norway and the Netherlands) or the eldest son of the current monarch (United Kingdom, Spain, etc) fills this role, but in Arab monarchies, for example, succession rules may differ and a Crown Prince may gain the title on 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 seldomly an official title in the European monarchies. Currently, only the heirs to the Scandinavian monarchies officially bear the title. Instead, many monarchies use special titles