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senior partner UK US noun [C]
► a member of a group of people, companies, countries, etc. that has more power and influence than other members: »
When the US and Japan discuss rules for financial services, Japan is now almost the senior partner.
»It was clear who the senior partner in the arrangement was.
► one of the people who has reached the top rank in a partnership: »
She is a senior partner in a New York law firm.
Financial and business terms. 2012.