An heir apparent is someone who stands the right to inherit as long as she or he outlives someone, assuming that there are no extreme extenuating circumstances. This term is most commonly used in references to hereditary titles, which operate under complex systems of inheritance. A related term, “heir presumptive,” refers to someone who would probably inherit in the event of someone's death, but could be displaced at any time.
Inherited titles are usually passed down under the system of primogeniture. In this system, the first born inherits everything. By convention, primogeniture was historically restricted to male children, which meant that if a king and queen had four daughters followed by a single son, for example, the son would be the heir apparent despite the fact that he would be younger than his sisters. Several nations have opted to change to a system of absolute primogeniture, in which inheritance laws ignore gender.
The heir apparent is the person who has the right to the title under the laws of primogeniture. In a nation with absolute primogeniture, this would be the first child of the couple which holds the title. If a couple failed to have children, they could have heirs presumptive in the form of siblings and the children of siblings, as well as among more distance relatives. These heirs could be usurped by an heir apparent, and in some regions, the law even required a waiting period to determine whether or not widows were pregnant before surrendering titles to heirs presumptive. If a widow was pregnant, her child would be the heir apparent, and such women sometimes acted as regents to hold a title until their children came of age.
In addition to inheriting the title, the heir apparent also inherits the estate. By convention, it is common for people to make settlements in life to ensure that other siblings are provided for, and once an heir apparent inherits a title, she or he may also make settlements on siblings and other relatives. However, the holder of a title and estate can opt to disinherit someone by making no provisions for her or his welfare.
Historically, disputes over inheritance of titles could become extremely bitter. Most nations today which retain hereditary titles and nobility use these titles primarily ceremonially. Historically, members of the nobility, especially the monarch, had absolute power and as a result competition to inherit was ferocious. People were not above killing to place themselves in a stronger position to inherit the throne. Being the heir apparent could actually be quite dangerous as many potential presumptive heirs would have had a strong interest in eliminating competition.