18 (number)

18 (eighteen) is the natural number following 17 and preceding 19.

In mathematics

Eighteen is the tenth
9. Three of these divisors (3, 6 and 9) add up to 18, hence 18 is a semiperfect number.^[1]
Eighteen is the first inverted square-prime of the form p·q².

In base ten, it is a Harshad number.
It is an
coprime
to this sum.
It is the number of one-sided pentominoes.
It is the only number where the sum of its written digits in
base 10
(1+8 = 9) is equal to half of itself (18/2 = 9).
It is a Fine number.^[2]

The Hebrew word for "life" is חי (chai), which has a numerical value of 18. Consequently, the custom has arisen in Jewish circles to give donations and monetary gifts in multiples of 18 as an expression of blessing for long life.^[3]
In
Chupah (marriage canopy)). (See Coming of age, Age of majority
).

Shemoneh Esrei (sh'MOH-nuh ES-ray) is a prayer that is the center of any Jewish religious service. Its name means "eighteen". The prayer is also known as the Amidah.
In
blood relative
.
Joseph Heller's novel Catch-22 was originally named Catch-18 because of the Hebrew meaning of the number, but was amended to the published title to avoid confusion with another war novel, Mila 18.^[4]
There are 18 chapters in the
Kuru side, 7 on the Pandava). The war itself lasts for 18 days. In the other Hindu epic, the Ramayana, the war between Rama
and the demons also lasted 18 days.

In
Babism the first 18 disciples of the Báb were known as the Letters of the Living
.

In
Chinese tradition, 18 is pronounced 十八; shí bā and is considered a lucky number due to similarity with 實發; shì fā 'definitely get rich', 'to get rich for sure'.^[5]

According to applications of numerology in Judaism, the letters of the word chai ("living") add up to 18. Thus, 18 is considered a lucky number and many gifts for B'nai Mitzvot and weddings are in $18 increments.^[6]

In most countries, 18 is the

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 aged over 18.

In association football (soccer), "the 18" is a slang term for the penalty area.
In Australian rules football (except in AFL Women's), each team has 18 players on the field during play.
There are 18 holes on a regulation golf course.
In Nippon Professional Baseball (Japanese baseball), No.18 is known as Ace number.