An heir apparent, sometimes heiress apparent, (however, heir is an equally valid term for a man, woman or child) is a person who is first in a line of succession and cannot be displaced from inheriting by the birth of another person. An heir presumptive, by contrast, is someone who is first in line to inherit a title but who can be displaced by the birth of a more eligible heir.
Today these terms most commonly describe heirs to hereditary titles, but they may also be accorded with a more specific substantive title, such as Prince of Orange in the Netherlands, Duke of Brabant in Belgium, Prince of Asturias in Spain or Prince of Wales in the United Kingdom; former titles include Dauphin of France during the Kingdom of France and Tsesarevich in Imperial Russia.
The term is also used metaphorically to indicate an "anointed" successor to any position of power, e.g. a political or corporate leader.
This article primarily describes the term heir apparent in a hereditary system regulated by laws of primogeniture —it may be less applicable to cases where a monarch has a say in naming the heir, performed either while alive, e.g. crowning the heir as a rex iunior, or through the monarch's will.