Eighteenth means number 18 in a sequence or order. When you finish eighteenth in a spelling bee, seventeen people placed ahead of you. If your birthday falls on the eighteenth of the month, it comes after the seventeenth and before the nineteenth.

The word describes position or rank in a series. A runner who crosses the finish line eighteenth didn't win the race, but they completed it ahead of everyone who came after them. In a long book, the eighteenth chapter falls near the middle if there are thirty or forty chapters total.

You might also use eighteenth to describe one part out of eighteen equal pieces. If you slice a pizza into eighteen pieces and take one slice, you're taking one eighteenth of the pizza. In fractions, this is written as 1/18.

The eighteenth century refers to the years 1701-1800, a period that saw the American Revolution and the rise of classical music composers like Mozart and Beethoven. People often call something eighteenth-century when describing the style, customs, or inventions from that era, like the elegant furniture or powdered wigs popular during that time.