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Related to éminence grise: gray eminence
Someone who is powerful in a particular area without having an official title. The phrase, French for "grey eminence," originally applied to one of Cardinal Richelieu's advisors, a monk who wore a grey robe. A: "Why should I worry about what Bob thinks? He doesn't even have a title!" B: "Yes, but he's the éminence grise of the firm, and the partners really value his opinion."
See also: eminence