The original concept of a power behind the throne was a Medieval-era figure of speech referring to the fact that the king's policies could be set by a counselor not seated in the throne but standing behind it—perhaps whispering in the king's ear—out of common sight.
Good historical examples of a "power behind the throne" include Diego Portales of Chile and General Hideki Tojo of Japan, who was influential in the military's actions leading up to World War II and later became Prime Minister of Japan in 1941. He served under Emperor Hirohito, who ruled as a deified figurehead. Another example is the rule of Pol Pot in Cambodia from 1975-1978, who led the Khmer Rouge to victory following a devastating civil war. King Norodom Sihanouk returned to reign over Cambodia, but held no executive power. In the United States, Edith Wilson - the second wife of President Woodrow Wilson - took over many of the routine duties and details of the government after her husband had been incapacitated by a stroke.
Another modern example was Deng Xiaoping in China, who was recognized as China's paramount leader without holding the position of either General Secretary or President. In Latin America, a good example was Doctor Joseph Marie Cordoba Montoya during the Presidency of Carlos Salinas de Gortari (1988-1994). Dr. Cordoba Montoya, a French naturalized Mexican, was the Head of the Office of the Presidency. Another example in the same region is the one of the former general Manuel Noriega, who was the military leader and the de facto chief of state of Panama from 1983 to 1989.
A related term is éminence grise (French: "gray eminence"), a powerful advisor or decision-maker who operates secretly or otherwise unofficially. This phrase originally referred to Cardinal de Richelieu's right hand man, François Leclerc du Tremblay (also known as the Père Joseph), a Capuchin friar who wore grey robes. Because the Cardinal de Richelieu, the power behind the throne of King Louis XIII of France, as a Catholic cardinal was styled Son Eminence ("His Eminence"), his alter ego Père Joseph was called l'éminence grise (which is also the English title of his biography by Aldous Huxley).
In sporting politics, a good example of this concept is that of Johan Cruyff's direct influence at FC Barcelona as a consultant, despite holding no official title whatsoever.