Use the noun doyen to describe the person who has been part of a group the longest, such as Mr. Thomas, the doyen of the accounting department. He is the doyen because everyone else joined after he did.

Generally speaking, the doyen isn't just the person who has been there the longest — the doyen is also an expert, perhaps in a specialized area that the others don't know as much about. A doyen is distinguished in a way that earns leadership status, as indicated by the word's origin in the Old French word deien, which comes from the Latin word decanus, or "head of ten monks in a monastery" or "commander of ten soldiers."

Definitions of doyen
  1. noun
    a man who is the senior member of a group
    synonyms: dean
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    type of:
    elder, senior
    a person who is older than you are
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