Meaning of “number one” in the English Dictionary

"number one" in British English

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number onenoun [ U ]

uk /ˌnʌm.bə ˈwʌn/ us /ˌnʌm.bɚ ˈwʌn/

(Definition of “number one” from the Cambridge Advanced Learner’s Dictionary & Thesaurus © Cambridge University Press)

"number one" in American English

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number onenoun [ C ]

us /ˈnʌm·bər ˈwʌn/

the best or highest ranking person or thing:

She’s still number one in tennis.

(Definition of “number one” from the Cambridge Academic Content Dictionary © Cambridge University Press)

"number one" in Business English

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number onenoun [ U ]

uk us

informal the person or thing that is the best or has the most important position in an organization, group, market, etc.:

They ran an aggressive advertising campaign as part of their plan to become number one in the UK market.
Sustainable success is our number one priority.

(Definition of “number one” from the Cambridge Business English Dictionary © Cambridge University Press)