Prince Regent

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 (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.

In the English language the title Prince Regent is most commonly associated with George IV, who held the style HRH The Prince Regent during 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. Nash, under the patronage of the HRH 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.

This period is known as the British Regency, or just the Regency.

The title was conferred by the Regency Act on February 5, 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.

...
Wikipedia