A queen consort is the wife and consort of a reigning king.
In contrast the husband of a reigning queen is (usually) not called "king consort"; rather, he is popularly called "prince consort". In the British system, a male consort does not automatically receive the title of "prince" until he is so created by the sovereign.
In general the consorts of monarchs have no constitutional status or power; they have merely the title.
There are a few cases in which a married couple ruled a kingdom jointly: Ferdinand II of Aragon and his wife Isabella, in her own right Isabella I of Castile, ruled their kingdoms as one dominion. Ferdinand was also called Ferdinand V of Castile. However, the two kingdoms would not be de jure united until the monarchs' grandson Charles V, Holy Roman Emperor, acceded to both thrones as Charles I of Spain.
The daughter of James II of England, Queen Mary II, married William of Orange; although Mary was the heiress to the throne, she and William chose to reign together and were made co-monarchs by Parliament, with William becoming king.
There have also been a number of cases when the queen consort of a deceased king (the Queen Dowager or Queen Mother) had served as regent while her child, the heir to the throne, was still a minor—for example, Catherine de Medici.
Besides these examples, there have been many cases of queen consorts being shrewd stateswomen and, albeit unofficially, being among the king's major advisors. In some cases, the queen consort has been the chief power behind the throne; example Henrietta Maria.
A notable exception to this rule is that of Camilla, Duchess of Cornwall, wife of Charles, Prince of Wales. It was announced that, in the event of Charles's ascent to the British throne, Camilla would assume the title of Princess Consort, not Queen Consort (although the legality of this under existing statutes has been called into question).
Examples of queens consort
Because queens consort lack an ordinal with which to distinguish between them, many historical texts and encyclopedias refer to deceased consorts by their pre-marital or maiden name or title, not by their marital royal title.