Source : Webster's Revised Unabridged Dictionary (1913)

Eightieth \Eight"i*eth\, a. [From {Eighty}.]
   1. The next in order after seventy-ninth.

   2. Consisting of one of eighty equal parts or divisions.

Eightieth \Eight"i*eth\, n.
   The quotient of a unit divided by eighty; one of eighty equal
   parts.

Source : WordNet®

eightieth
     adj : the ordinal number of eighty in counting order [syn: {80th}]
     n : position 80 in a countable series of things