Meaning of “twenty-first” in the English Dictionary

"twenty-first" in British English

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twenty-firstordinal number

uk /ˌtwen.tiˈfɜːst/ us /ˌtwen.t̬iˈfɝːst/

twenty-firstnoun [ C ]

uk /ˌtwen.tiˈfɜːst/ us /ˌtwen.t̬iˈfɝːst/ also twenty-first birthday UK

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 Learner’s Dictionary & Thesaurus © Cambridge University Press)