The Collaborative International Dictionary of English v.0.48:

Eightieth \Eight"i*eth\, a. [From Eighty.]
   1. The next in order after seventy-ninth.
      [1913 Webster]

   2. Consisting of one of eighty equal parts or divisions.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Eightieth \Eight"i*eth\, n.
   The quotient of a unit divided by eighty; one of eighty equal
   parts.
   [1913 Webster]

WordNet (r) 3.0 (2006):

eightieth
    adj 1: the ordinal number of eighty in counting order [syn:
           eightieth, 80th]
    n 1: position 80 in a countable series of things