1) Being one of 80 equal parts 2) Next after the seventy-ninth 3) Ordinal 
• (a.) The next in order after seventy-ninth. • (n.) The quotient of a unit divided by eighty; one of eighty equal parts. • (a.) Consisting of one of eighty equal parts or divisions.
Eight'i·eth adjective [ From Eighty .] 1. The next in order after seventy-ninth. 2. Consisting of one of eighty equal parts or divisions. 
[adj] - the ordinal number of eighty in counting order 2. [n] - position 80 in a countable series of things
80th adjective the ordinal number of eighty in counting order
next after the seventy-ninth; being the ordinal number for 80. · being one of 80 equal parts.