eighty-first I. \| ̷ ̷ ̷ ̷| ̷ ̷\adjective 1.: being number 81 in a countable series < the eighty-first day > — see number table 2.: being one of 81 equal parts into which something is divisible < an eighty-first share of the money > II. noun 1.: number 81 in a countable series 2.: the quotient of a unit divided by 81 : one of 81 equal parts of something < one eighty-first of the total >