Historically, the Privy Council was the name given to the group of ministers who acted as chief advisers to the King or Queen.
As the power of the monarch declined, the cabinet replaced the Privy Council as the UK's senior decision-making body.
Today the Privy Council's duties are largely formal, meeting to approve decisions that have been taken elsewhere and making appointments to offices of the crown.
But it can exercise executive, legal and legislative powers in certain circumstances.
It has about 550 members, including all cabinet members past and present, the leaders of all the main parties, the Speaker, Archbishops, senior judges and some other important public figures.
Members are entitled to use the prefix "Right Honourable".
The Privy Council meets in the presence of the Queen, or one of her Counsellors in State if she cannot attend, and the Lord President of the Council, a senior government minister.
Usually, just a handful of Privy Councillors are called to Buckingham Palace for each meeting.
But on rare ceremonial occasions such as the proclamation of a new monarch, larger meetings are convened.
Powers and privileges
Until recently, a Privy Councillor would have taken precedence over other backbench MPs when the Speaker chooses who should be allowed to contribute to a debate - but this rule has now been abandoned.
The Privy Council retains the power to alter the statute book, but only in certain circumstances.
It does this by issuing orders in council, which can be used, for example, to transfer powers between government departments.
Technically, the Queen issues such orders "by and with the advice of Her Privy Council".
In reality, most important changes are originated by government ministers using powers given to them by acts of Parliament - this is a type of delegated legislation.
But the Privy Council can exercise the right to enshrine in law executive policies independently of the parliamentary process, like the government's decision to continue to block the Chagos islanders from returning home in 2004.
The Privy Council also has a judicial arm, acting as the highest court of appeal in cases originating in the Channel Isles, the Isle of Man, some Commonwealth countries and all British overseas territories - as well as hearing appeals from some UK professional disciplinary bodies.
It also oversees the administration of all organisations that have been incorporated under a royal charter.
There are more than 900 such organisations, including many UK universities, the City of London, and the BBC.