An heir apparent is a person who is first in the 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 an heir presumptive.

Today these terms most commonly describe heirs to hereditary titles (e.g. titles of nobility) or offices, especially when only inheritable by a single person. Most monarchies refer to the heir apparent of their thrones with the descriptive term of crown prince or crown princess, 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 (also granted to heirs presumptive), or the Prince of Wales in England and Wales; former titles include Dauphin in the Kingdom of France, and Tsesarevich in Imperial Russia.

The term is also applied metaphorically to an expected 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).

In a hereditary system governed by some form of primogeniture, an heir apparent is easily identifiable as the person whose position as first in the line of succession to a title or office is secure, regardless of future births. An heir presumptive, by contrast, can always be "bumped down" in the succession by the birth of somebody more closely related in a legal sense (according to that form of primogeniture) to the current title-holder.

The clearest example occurs in the case of a childless bearer of a hereditary title that can only be inherited by one person. If at any time the title bearer were to produce children, those children would rank ahead of any person who had formerly been heir presumptive.

Many legal systems assume childbirth is always possible regardless of age or health. In such circumstances a person may be, in a practical sense, the heir apparent but still, legally speaking, heir presumptive. Indeed, when Queen Victoria succeeded her uncle King William IV, the wording of the proclamation even gave as a caveat:

...saving the rights of any issue of his late Majesty King William IV, which may be born of his late Majesty's consort.