18 (eighteen) is the natural number following 17 and preceding 19. It is an even composite number.

Mathematics

18 is a semiperfect number and an abundant number. It is a largely composite number, as it has 6 divisors and no smaller number has more than 6 divisors. There are 18 one-sided pentominoes. In the classification of finite simple groups, there are 18 infinite families of groups.

In science

Chemistry

In religion and literature

As lucky or unlucky number

Age 18

In most countries, 18 is the age of majority, in which a minor becomes a legal adult. It is also the voting age, marriageable age, drinking age and smoking age in most countries, though sometimes these ages are different than the age of majority. Many websites restrict adult content to visitors who claim to be at least 18 years old.

In sports