fifty-first I. \| ̷ ̷ ̷ ̷| ̷ ̷\adjective 1.: being number 51 in a countable series < the fifty-first day > — see number table 2.: being one of 51 equal parts into which something is divisible < a fifty-first share of the money > II. noun 1.: number 51 in a countable series 2.: the quotient of a unit divided by 51 : one of 51 equal parts of something < one fifty-first of the total >