The ordinal number matching the number 80 in a series.(noun)
One of 80 equal parts.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "eightieth" in a sentence
"Lady Bird Johnson celebrated her eightieth birthday in her red evening gown."
"What drives you—just months shy of your eightieth birthday—to continue to work so hard?"
"I know Apache and he's no Apache., but he looked so white, you'd have taken him for eightieth generation Bostonian."