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English definition of “twenty-first”

twenty-first

ordinal number uk   /ˌtwen.tiˈfɜːst/ us    /-t̬iˈfɝːst/
21st written as a word: the twenty-first century

twenty-first

noun [C] uk   /ˌtwen.tiˈfɜːst/ us    /-t̬iˈfɝːst/ ( also twenty-first birthday)
the day on which a person reaches the age of 21 and is traditionally said, in Western societies, to become an adult: It's Jamie's twenty-first next Friday. Her dad gave her a car for her twenty-first.
(Definition of twenty-first from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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