Meaning of “thirtieth” in the English Dictionary

"thirtieth" in American English

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thirtiethadjective, adverb, noun [ C ]

us /ˈθɜr·t̬i·ɪθ/

(in the position of) the number 30 in a series; 30th:

Ashley’s having a party to celebrate her thirtieth birthday.
[ C ] Today is the thirtieth of September.

A thirtieth is one of thirty equal parts of something.

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