A dean; the senior member of a group.

Dictionary.com reports that this word likely stems from the Old French doien, from the Late Latin decanus, meaning "chief of ten." Dean has a related etymology.

Thanks for Gorgonzola for pointing out that doyenne is often used to refer to a woman who is the eldest or senior-ranking memor of a group, or simply a female specialist in her field (often used as synonymous with "guru," and often used in reference to Martha Stewart).