Eightieth (adjective)

1. The ordinal form of the number eight, indicating a position or order in a series.
2. The next number after the 79th in counting order.

Origin:

From the combining form of the number "eight" + "ieth", from the old english "-tigotha" meaning "tenth", a suffix used to form ordinal numbers.

Examples:

1. He turned 80 on his eightieth birthday.
2. Today is my eightieth day of working out.
3. The eightieth floor is the top floor of the building.
4. The eightieth president of the United States.
5. The eightieth anniversary of the company was a big celebration.