Eightieth means the number 80 in a series, or one of 80 equal parts of something.

When runners finish a race, the eightieth person crosses the finish line after 79 others. When you celebrate your grandmother's eightieth birthday, she's lived eight full decades. If you're reading the eightieth page of a book, you've made it through 79 pages already.

The word can also describe a fraction: one eightieth of something means dividing it into 80 equal pieces and taking just one. If 80 students equally share the cost of a class trip, each pays one eightieth of the total cost.

You write eightieth as 80th using numerals. Like other ordinal numbers (first, second, third), it shows position or order rather than quantity. While eighty tells you how many (80 pencils), eightieth tells you where something falls in a sequence (the 80th pencil in line).