Eighteen is the number that comes after seventeen and before nineteen. It's written as 18 in numerals and represents one more than seventeen or two more than sixteen.

In many places, turning eighteen years old marks an important transition into legal adulthood. At eighteen, people often gain new rights and responsibilities, such as being able to vote in elections, serve on juries, and sign contracts without parental permission. In the United States, many eighteen-year-olds can join the military, get married without parental consent, and may be tried as adults in criminal courts. This special status makes turning eighteen a major milestone that families often celebrate with parties or special gifts.

The number eighteen appears in many contexts. A golf course typically has eighteen holes. In some sports leagues, players wear jersey number 18. When counting, eighteen is part of the teens, that group of numbers from thirteen through nineteen that all end in “teen.”

Mathematically, eighteen is an even number (divisible by 2) and a composite number (it has factors other than 1 and itself: 1, 2, 3, 6, 9, and 18). It's also the sum of three consecutive numbers: 5 + 6 + 7 = 18.