"A prince regent, or prince-regent, is a prince who rules a monarchy as regent instead of a monarch, e.g., as a result of the Sovereign´s incapacity ...
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(minority or illness) or absence (remoteness, such as exile or long voyage, or simply no incumbent). While the term itself can have the generic meaning and refer to any prince who fills the role of regent, historically it has mainly been used to describe a small number of individual princes who were regents." - (en.wikipedia.org 28.09.2019)