ninety-first 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