An éminence grise (French for "grey eminence") is a powerful advisor or decision-maker who operates secretly or unofficially. This phrase originally referred to François Leclerc du Tremblay, the right-hand man of Cardinal Richelieu. Leclerc was a Capuchin friar who wore grey, or rather brown, robes. Brown or light brown (now called "beige") was called grey in that era. The phrase "His Eminence" is used to address or reference a Cardinal in the Roman Catholic Church. Although Leclerc du Tremblay never achieved that rank, it is probable that those around him may have addressed him thus in reference to the considerable influence this "grey friar" held over "his Eminence the Cardinal". Aldous Huxley wrote an English biography of Leclerc entitled Grey Eminence, and there is also an 1873 painting by Jean-Léon Gérôme, "L'Éminence Grise," which depicts him descending the grand staircase of the Palais Cardinal.
A grey eminence may alternatively mean an elderly (grey-haired) individual who is eminent for accomplishments in the past, but today acts as an advisor rather than a principal actor, and may be politically influential as a consequence of his honored status. For example a distingushed retired physics professor; a politician who retired with a good reputation; etc.