Ninetieth means number ninety in a sequence, coming after the eighty-ninth and before the ninety-first. When your great-grandmother celebrates her ninetieth birthday, she's marking her 90th year of life.

You might encounter this word when reading historical timelines: “The telephone was invented in the ninetieth year of the nineteenth century” means 1890. In sports, a soccer player who scores in the ninetieth minute is scoring near the very end of the match.

The word can also describe one of ninety equal parts of something. If you divide a pizza into ninety slices, each slice would be one-ninetieth of the whole pizza. When written as a fraction, that's 1/90.

Like other ordinal numbers (first, second, third), ninetieth helps us describe position or order. While you'll rarely need to divide something into ninety parts, you'll use this word to mark significant milestones, especially birthdays or anniversaries that celebrate nine decades of something important.