Noun
(French) a person who exercises power or influence in certain areas without holding an official position
Source: WordNetthe President's wife is an eminence grise in matters of education Source: Internet
Despite a penchant for poetry and art, she is no less formidable than her husband and believed by many to be the eminence grise behind him. Source: Internet
"He is active from the background, an eminence grise," the source said. Source: Internet