Eightieth/ˈeɪtiəθ/ Definition 1 of 2Pro Tip 1/3Milestone Events Often used to mark significant occasions, especially anniversaries and celebrations. Her family threw a big party for her eightieth birthday. Pro Tip 2/3Ordinal Form Used in sequence to indicate specific position, not quantity. He was the eightieth runner to cross the finish line. Pro Tip 3/3Historical Context Connects events to a specific place in history, good for understanding timelines. The eightieth congress of the United States convened in 1947.