ninety-first

adjective

Definition of ninety-first

 (Entry 1 of 2)

1 : being number 91 in a countable series the ninety-first day — see Table of Numbers
2 : being one of 91 equal parts into which something is divisible a ninety-first share of money

ninety-first

noun
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Definition of ninety-first (Entry 2 of 2)

1 : number 91 in a countable series
2 : the quotient of a unit divided by 91 : one of 91 equal parts of something