I. | ̷ ̷ ̷ ̷| ̷ ̷ adjective
1. : being number 91 in a countable series
the ninety-first day
— see number table
2. : being one of 91 equal parts into which something is divisible
a ninety-first share of money
II. noun
1. : number 91 in a countable series
2. : the quotient of a unit divided by 91 : one of 91 equal parts of something