n. A union (not merger) between two (or more) states through the person of a shared monarch, who is head of state of each, under separate constitutions, although dyastic policy aims to continue this union after each succession to the thrones.
A personal union is the combination of two or more states that have the same monarch while their boundaries, laws, and interests remain distinct. It differs from a federation in that each constituent state has an independent government, whereas a unitary state is united by a central government. The ruler in a personal union does not need to be a hereditary monarch.
Personal unions can arise for several reasons, ranging from coincidence (a woman who is already married to a king becomes queen regnant, and their child inherits the crown of both countries) to virtual annexation (where a personal union sometimes was seen as a means of preventing uprisings). They can also be codified (i.e., the constitutions of the states clearly express that they shall share the same person as head of state) or non-codified, in which case they can easily be broken (e.g., by the death of the monarch when the two states have different succession laws).
The Commonwealth realms, not listed below, are independent states that share the same person as monarch.
Because presidents of republics are ordinarily chosen from within the citizens of the state in question, the concept of personal union has almost never crossed over from monarchies into republics. A rare example of a republican president with a personal union, however, is the President of France. Even though France is a republic with a president and not a monarchy, it is nevertheless in personal union with the neighboring nominal monarchy (non-hereditary) of Andorra.