Distinction between heir apparent and heir presumptive from the
http://dictionary.laborlawtalk.com/heir_apparent:
heir apparent - an heir whose right to an inheritance cannot be defeated if that person outlives the ancestor
Antonyms: heir presumptive - a person who expects to inherit but whose right can be defeated by the birth of a nearer relative
According to the order of succession of many monarchies, the heir apparent is a 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.
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According to this definition, neither Charles or William is the heir presumptive because no one else can assume their right to the throne ahead of them if they live long enough. If Charles outlives the Queen, he will be King. If William outlives the Queen and Charles, he will be King. Also neither one of them is the heir. According to this definition, the Queen is the heir because she has inherited the throne.
I'm only used to seeing the immediate heir (in this case Charles) called the heir apparent but under this definition, I suppose William could be also the heir apparent.
Chrissy, do you know if both William and Charles could be the heir apparent or is there only one at one time?