Eminence grise is a noun that refers to a person who unofficially has great power.

Our word of the day comes directly from a Latin phrase that once referred to a royal confidant of a powerful figure. More recently it has been used to refer to the elder member of a field or profession — one who still has power despite no longer holding a title.

Horace may be the company’s president, but believe me, Antonius is the one who calls the shots. He is after all, the eminence grise of the company and nobody would dare challenge his wisdom.