Prince regent

A prince regent or princess regent is a prince or princess who, due to their position in the line of succession, rules a monarchy as regent in the stead of a monarch, e.g., as a result of the sovereign's incapacity (minority or illness) or absence (e.g., by remoteness, such as exile or long voyage, or the absence of an incumbent).
While the term itself can have the generic meaning and refer to any prince or princess who fills the role of regent, historically it has mainly been used to describe a small number of individual princes and princesses who were regents of non-principalities.
In the English language the title Prince Regent is most commonly associated with George IV, who held the style His Royal Highness The Prince Regent during the Regency era due to the incapacity (by dint of mental illness) of his father, George III (see Regent for other regents). Regent's Park, Regent Street and Regent's Canal (which he commissioned) in London, were all named in honour of him. The architect John Nash, under the patronage of HRH The Prince Regent, planned a palatial summer residence for the prince, 50 detached villas in a parkland setting and elegant terraces around the exterior of the park. This was all part of an ambitious plan, to develop The Regent's Park and lay out an elegant new street, Regent's Street, to link it to St James's Park and the prince's London residence, Carlton House. Regent Terrace in Edinburgh is also named after the Prince Regent, who visited the area in 1822.
This period is known as the British Regency, or just the Regency.
The title was conferred by the Regency Act on 5 February 1811. Subject to certain limitations for a period, the prince regent was able to exercise the full powers of the King. The precedent of the Regency Crisis of 1788 (from which George III recovered before it was necessary to appoint a regent) was followed. The Prince of Wales continued as regent until his father's death in 1820, when he became George IV.
In Germany, the title Prinzregent (literally prince regent) is most commonly associated with Prince Luitpold of Bavaria, who served as regent for two of his nephews, King Ludwig II of Bavaria, who was declared mentally incompetent in 1886, and King Otto of Bavaria (who had been declared insane in 1875) from 1886 until 1912.
The years of Luitpold's regency were marked by tremendous artistic and cultural activity in Bavaria, where they are known after the regencies as the Prinzregentenjahre or the Prinzregentenzeit. Numerous streets in Bavarian cities and towns are called Prinzregentenstraße. Many institutions are named in Luitpold's honour, e.g., the Prinzregententheater in Munich. Prinzregententorte is a multi-layered cake with chocolate butter cream named in Luitpold's honour.
At Luitpold's death in 1912, his son Prince Ludwig succeeded as prince regent. Ludwig held the title for less than a year, since the Bavarian Legislature decided to set aside his cousin King Otto I of Bavaria and recognize him as king.
Kiril, Prince of Preslav was appointed head of a regency council by the Bulgarian parliament following the death of his brother, Tsar Boris III on 28 August 1943, to act as Head of State until the late Tsar's son and successor, Tsar Simeon II, reached the age of 18 years. On 5 September 1944 the Soviet Union declared war on the Kingdom of Bulgaria and on 8 September Soviet armies crossed the Romanian border and occupied the country. On 1 February 1945 the prince regent Kyril, and the two other former regents – Professor Bogdan Filov and General Nikola Mikhov – as well as a range of former cabinet ministers, royal advisors and 67 MPs, were executed.
The heir-apparent or heir-presumptive to the grand duke of Luxembourg may be titled prince-lieutenant ('prince deputy') during a period in which the incumbent remains formally on the grand ducal throne, but (progressively, most) functions of the crown are performed by the 'monarch apprentice', as prince Jean did 4 May 1961 – 12 November 1964 in the last years of his mother Charlotte's reign until she abdicated and he succeeded to the grand ducal throne (she lived until 1985), and Jean's own son prince Henri 3 March 1998 – 7 October 2000 until his father abdicated and he succeeded (Jean lived until 2019).
If a king is unable to perform his duties then his consort or the queen mother may act for him as a Queen Regent. Queen mothers have acted in this way in the Kingdom of Eswatini.