thir•ty-firstPronunciation: (thûr'tē-fûrst'), [key]
next after the thirtieth; being the ordinal number for 31.
being one of 31 equal parts.
a thirty-first part, esp. of one (1/31
the thirty-first member of a series.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.