18 (number)

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

Mathematics

The 18-electron rule is a rule of thumb in transition metal chemistry for characterising and predicting the stability of metal complexes.
In religion and literature

The Hebrew word for "life" is חי, which has a numerical value of 18. Consequently, a custom has arisen in Jewish circles to give donations and monetary gifts in multiples of 18 as an expression of blessing for long life.
In Judaism, in the Talmud; Pirkei Avot, Rabbi Yehudah ben Teime gives the age of 18 as the appropriate age to get married..
The Amidah prayer is also known Shemoneh Esrei as because it has eighteen blessings.
Other
In Ancient Roman custom the number 18 can symbolise a blood relative.
In Hinduism, in the Mahabharata, the number appears in 18 books, the 18 chapters of the Bhagavad Gita, the 18 days of the Kurukshetra War, and the 18 Akshauhini divisions of armies. There are also 18 major and 18 minor Puranas.
As lucky or unlucky number
In Chinese tradition, 18 is pronounced p=shí bā and is considered a lucky number due to similarity with p=shì fā 'definitely get rich', 'to get rich for sure'.
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 the United States, 18 is the:
*Age for sexual consent in eleven states and under federal law.
*Minimum age to purchase firearms in thirty-eight states with the exception of handguns.
*Marriageable age without parental consent except for Nebraska, Mississippi, and Puerto Rico.
*The minimum age at which one can purchase, rent, or buy tickets to NC-17-rated films or buy video games with an Adults Only rating.
In the UK, 18 is the legal age to purchase a BBFC "18" rated film.
In Japan, 18 is the minimum age at which one can purchase, rent, or buy tickets to R18+ rated movies or buy video games with a Z rating, although 20 is their cultural age of adulthood.
In sports
In association football, "the 18" is a slang term for the penalty area.
In Nippon Professional Baseball, No.25 is known as the Ace number.