Heir Presumptive
An heir presumptive or heiress presumptive is the person provisionally scheduled to inherit a throne, peerage, or other hereditary honour, but whose position can be displaced (in legal terms, is "subject to divestiture") by the birth of an heir or heiress apparent
Heir apparent
An heir apparent or heiress apparent is a person who is first in line of succession and cannot be displaced from inheriting, except by a change in the rules of succession....

 or of a new heir presumptive with a better claim to the position in question. When lowercased, "heir presumptive" can refer generally to someone who is provisionally scheduled to inherit a title, position or possession, unless displaced by an heir apparent or another heir presumptive born further up the line of succession.