[determiner], [pronoun], [adjective], [adverb] [not gradable], nounShe's having a big party for her eightieth (birthday).They were/finished eightieth out of hundred.An eightieth is one of eighty equal parts of something.
EIGHTIETH
Meaning of EIGHTIETH in English
Cambridge English vocab. Кембриджский английский словарь. 2012