Princess
Princess is a regal rank and the feminine equivalent of prince (from Latin princeps, meaning principal citizen). Most often, the term has been used for the consort of a prince or for the daughters of a king or sovereign prince.
Princess as a substantive title
The titles of some princesses hold their titles are reigning monarchs of principalities. There have been fewer instances of reigning princesses than reigning princes as most principalities excluded women from inheriting the throne. Examples of princesses regnant have included Constance of Antioch, princess regnant of Antioch in the 12th century. As the President of France, an office for which women are eligible, is ex-officio co-Prince of Andorra, Andorra could theoretically be ruled by a co-Princess.
Princess as a courtesy title
Descendants of monarchs
For many centuries, the title "princess" was not regularly used for a monarch's daughter, who, In English, might simply be called "Lady". Old English had no female equivalent of "prince", "earl", or any royal or noble title aside from queen. Royal women were simply addressed or referred to as "The Lady [Firstname]". For example, Elizabeth and Mary, daughters of Henry VIII of England were often simply referred to as "the Ladies Elizabeth and Mary". This practice, however, was not consistent. In the marriage contract between Prince George of Denmark and Anne, daughter of James I of Great Britain, Anne is referred to as "The Princess Anne".