I. | ̷ ̷ ̷ ̷| ̷ ̷ adjective
1. : being number 31 in a countable series
the thirty-first day
— see number table
2. : being one of 31 equal parts into which anything is divisible
a thirty-first share of the money
II. noun
1. : number 31 in a countable series
the thirty-first of the month
2. : the quotient of a unit divided by 31 : one of 31 equal parts of anything
one thirty-first of the total