Prince or Princess of Asturias is the main substantive title used by the heir apparent, heiress apparent or heir presumptive to the throne of Spain. According to the Spanish Constitution of 1978:Article 57 [...]
2. The heir apparent, heiress apparent or presumptive, from birth or event that makes him such, will have the dignity of Prince of Asturias and other titles traditionally linked to the successor of the Crown of Spain.
Prince of Asturias
Imaginative portrait of King Henry III of Castile, by Calixto Ortega, 1848. He was the first Prince of Asturias, proclaimed in 1388.
Education of Prince John, by Salvador Martínez Cubells 1877. John was the only son of the Catholic Monarchs and heir of all their domains during his lifetime.
Cover of the first edition of the Political Constitution of the Spanish monarchy. Cádiz, 19 March 1812
An heir apparent or simply heir is a person who is first in an order of succession and cannot be displaced from inheriting by the birth of another person. A person who is first in the current order of succession but could be displaced by the birth of a more eligible heir is known as heir presumptive.
Throngs before the Imperial Palace in Japan awaiting the appearance of the Crown Prince Hirohito for the recent proclamation of his official recognition as the heir apparent to the Japanese Imperial Throne – New York Times, 1916.