ninetieth (adj., n.)

"next in order or rank after the eighty-ninth; an ordinal numeral corresponding to ninety; being one of ninety equal parts into which a whole is regarded as divided;" late 14c., nyntithe, from ninety + -th (1); a replacement or modification of Old English nigenteoþan.

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Definitions of ninetieth from WordNet

ninetieth (adj.)
the ordinal number of ninety in counting order;
Synonyms: th