Personal union

A personal union is a relationship of two or more entities that are considered separate, sovereign states, which, through established law, share the same person as their respective head of state. It is not to be confused with a federation, which internationally is considered as a single state. Personal unions can arise for very different reasons, ranging from near coincidence (a princess who is already married to a king becomes pregnant, 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 different succession rules).
See more at Wikipedia.org...