An Heir Apparent is a direct descendant of a monarch who is the heir to their throne, whose status as heir cannot be defeated by the birth of someone else who would then become the heir. An heir apparent differs from an Heir Presumptive in that, although an heir presumptive inherits the throne upon the death of the monarch, the right of the heir presumptive could be defeated by the birth of another person who would then be the heir apparent. For example, in Britain, if the monarch has a daughter and no sons, the daughter is the heiress presumptive, who becomes Queen if the monarch dies. But if the monarch later has a son, the son is then the heir apparent. In most monarchies, the monarch's oldest son, even if not the oldest child, becomes Heir Apparent. A growing number of monarchies however now allow the monarch's oldest child, irrespective of sex, to become Heir Apparent. Where that heir has died leaving children, their oldest child (or oldest son) becomes Heir Apparent.
It is an error to call the heir apparent simply the "heir." The heir is the person who currently owns the estate, having inherited it - in this case, the currently reigning monarch.