18 (eighteen) is the

natural number In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country"). Numbers used for counting are called ''Cardinal n ...

following 17 and preceding 19.

In mathematics

* Eighteen is a

composite number A composite number is a positive integer that can be formed by multiplying two smaller positive integers. Equivalently, it is a positive integer that has at least one divisor other than 1 and itself. Every positive integer is composite, prime, ...

, its

divisor In mathematics, a divisor of an integer n, also called a factor of n, is an integer m that may be multiplied by some integer to produce n. In this case, one also says that n is a multiple of m. An integer n is divisible or evenly divisible by ...

s being 1, 2, 3, 6 and 9. Three of these divisors (3, 6 and 9) add up to 18, hence 18 is a

semiperfect number In number theory, a semiperfect number or pseudoperfect number is a natural number ''n'' that is equal to the sum of all or some of its proper divisors. A semiperfect number that is equal to the sum of all its proper divisors is a perfect number. ...

. Eighteen is the first inverted square-prime of the form ''p''·''q''². * In

base ten The decimal numeral system (also called the base-ten positional numeral system and denary or decanary) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic numeral ...

, it is a

Harshad number In mathematics, a harshad number (or Niven number) in a given number base is an integer that is divisible by the sum of its digits when written in that base. Harshad numbers in base are also known as -harshad (or -Niven) numbers. Harshad numbers ...

. * It is an abundant number, as the sum of its proper divisors is greater than itself (1+2+3+6+9 = 21). It is known to be a solitary number, despite not being 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.

In science

Chemistry

* Eighteen is the

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...

argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

. * Group 18 of the

periodic table The periodic table, also known as the periodic table of the (chemical) elements, is a rows and columns arrangement of the chemical elements. It is widely used in chemistry, physics, and other sciences, and is generally seen as an icon of ch ...

is called the noble gases. * The 18-electron rule is a

rule of thumb In English, the phrase ''rule of thumb'' refers to an approximate method for doing something, based on practical experience rather than theory. This usage of the phrase can be traced back to the 17th century and has been associated with various t ...

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...

chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

for characterising and predicting the stability of

metal complexes A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

In religion and literature

* The

Hebrew Hebrew (; ; ) is a Northwest Semitic language of the Afroasiatic language family. Historically, it is one of the spoken languages of the Israelites and their longest-surviving descendants, the Jews and Samaritans. It was largely preserved ...

word for "life" is ('' 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. * In

Judaism Judaism ( he, ''Yahăḏūṯ'') is an Abrahamic, monotheistic, and ethnic religion comprising the collective religious, cultural, and legal tradition and civilization of the Jewish people. It has its roots as an organized religion in the ...

, in the

Talmud The Talmud (; he, , Talmūḏ) is the central text of Rabbinic Judaism and the primary source of Jewish religious law (''halakha'') and Jewish theology. Until the advent of modernity, in nearly all Jewish communities, the Talmud was the cente ...

;

Pirkei Avot Pirkei Avot ( he, פִּרְקֵי אָבוֹת; also transliterated as ''Pirqei Avoth'' or ''Pirkei Avos'' or ''Pirke Aboth''), which translates to English as Chapters of the Fathers, is a compilation of the ethics, ethical teachings and Maxim ...

(5:25), Rabbi Yehudah ben Teime gives the age of 18 as the appropriate age to get married (''"Ben shmonah esra lechupah"'', at eighteen years old to the

Chupah A ''chuppah'' ( he, חוּפָּה, pl. חוּפּוֹת, ''chuppot'', literally, "canopy" or "covering"), also huppah, chipe, chupah, or chuppa, is a canopy under which a Judaism, Jewish couple stand during their Jewish wedding, wedding cere ...

(marriage canopy)). (See

Coming of age Coming of age is a young person's transition from being a child to being an adult. The specific age at which this transition takes place varies between societies, as does the nature of the change. It can be a simple legal convention or can b ...

, 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 The ''Amidah Amuhduh'' ( he, תפילת העמידה, ''Tefilat HaAmidah'', 'The Standing Prayer'), also called the ''Shemoneh Esreh'' ( 'eighteen'), is the central prayer of the Jewish liturgy. Observant Jews recite the ''Amidah'' at each o ...

. * In

Ancient Roman In modern historiography, ancient Rome refers to Roman civilisation from the founding of the city of Rome in the 8th century BC to the collapse of the Western Roman Empire in the 5th century AD. It encompasses the Roman Kingdom (753–509 BC ...

custom the number 18 can symbolise a

blood relative Consanguinity ("blood relation", from Latin '' consanguinitas'') is the characteristic of having a kinship with another person (being descended from a common ancestor). Many jurisdictions have laws prohibiting people who are related by blood fr ...

. * Joseph Heller's novel ''

Catch-22 ''Catch-22'' is a satirical war novel by American author Joseph Heller. He began writing it in 1953; the novel was first published in 1961. Often cited as one of the most significant novels of the twentieth century, it uses a distinctive non-chr ...

'' 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 ''Mila 18'' is a novel by Leon Uris set in German-occupied Warsaw, Poland, before and during World War II. ''Mila 18'' debuted at #7 on ''The New York Times'' Best Seller List (the second-highest debut of any Uris novel ever, bested only by th ...

''.N James. ''The Early Composition History of Catch-22''. In Biographies of Books: The Compositional Histories of Notable American Writings, J Barbour, T Quirk (edi.) pp. 262-90. Columbia:

University of Missouri Press The University of Missouri Press is a university press operated by the University of Missouri in Columbia, Missouri and London, England; it was founded in 1958 primarily through the efforts of English professor William Peden. Many publications a ...

, 1996 * There are 18 chapters in the

Bhagavad Gita The Bhagavad Gita (; sa, श्रीमद्भगवद्गीता, lit=The Song by God, translit=śrīmadbhagavadgītā;), often referred to as the Gita (), is a 700- verse Hindu scripture that is part of the epic ''Mahabharata'' (c ...

, which is contained in the

Mahabharata The ''Mahābhārata'' ( ; sa, महाभारतम्, ', ) is one of the two major Sanskrit epics of ancient India in Hinduism, the other being the ''Rāmāyaṇa''. It narrates the struggle between two groups of cousins in the Kuruk ...

, which has 18 books. The

Kurukshetra War The Kurukshetra War ( sa, कुरुक्षेत्र युद्ध ), also called the Mahabharata War, is a war described in the ''Mahabharata ( sa, महाभारत )''. The conflict arose from a dynastic succession struggle be ...

which the

epic Epic commonly refers to: * Epic poetry, a long narrative poem celebrating heroic deeds and events significant to a culture or nation * Epic film, a genre of film with heroic elements Epic or EPIC may also refer to: Arts, entertainment, and medi ...

depicts, is between 18 armies (11 on the

Kuru Kuru may refer to: Anthropology and history * Kuru (disease), a type of transmissible spongiform encephalopathy associated with the cannibalistic funeral practices of the Fore people * Kuru (mythology), part of Meithei mythology * Kuru Kingdom, ...

side, 7 on the

Pandava The Pandavas (Sanskrit: पाण्डव, IAST: Pāṇḍava) refers to the five legendary brothers— Yudhishthira, Bhima, Arjuna, Nakula and Sahadeva—who are the central characters of the Hindu epic ''Mahabharata''. They are acknowledg ...

). The war itself lasts for 18 days. In the other Hindu epic, the

Ramayana The ''Rāmāyana'' (; sa, रामायणम्, ) is a Sanskrit literature, Sanskrit Indian epic poetry, epic composed over a period of nearly a millennium, with scholars' estimates for the earliest stage of the text ranging from the 8th ...

, the war between

Rama Rama (; ), Ram, Raman or Ramar, also known as Ramachandra (; , ), is a major deity in Hinduism. He is the seventh and one of the most popular '' avatars'' of Vishnu. In Rama-centric traditions of Hinduism, he is considered the Supreme Bein ...

and the demons also lasted 18 days. * In Babism the first 18 disciples of the

Báb The Báb (b. ʿAlí Muḥammad; 20 October 1819 – 9 July 1850), was the messianic founder of Bábism, and one of the central figures of the Baháʼí Faith. He was a merchant from Shiraz in Qajar Iran who, in 1844 at the age of 25, claimed ...

were known as the Letters of the Living.

As lucky or unlucky number

* In

Chinese tradition Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of va ...

, 18 is pronounced and is considered a lucky number due to similarity with 'definitely get rich', 'to get rich for sure'. *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.

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 Marriageable age (or marriage age) is the general age, as a legal age or as the minimum age subject to parental, religious or other forms of social approval, at which a person is legitimately allowed for marriage. Age and other prerequisites to ...

, drinking age and

smoking age smoking age is the minimum legal age required to purchase or use tobacco products. Most countries have laws that forbid sale of tobacco products to persons younger than certain ages, usually the age of majority. This article does not discuss l ...

in most countries, though sometimes these ages are different than the age of majority. Many

websites A website (also written as a web site) is a collection of web pages and related content that is identified by a common domain name and published on at least one web server. Examples of notable websites are Google, Facebook, Amazon, and Wikipe ...

restrict adult content to visitors who claim to be aged over 18. *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 (21 under federal law). **

Marriageable age Marriageable age (or marriage age) is the general age, as a legal age or as the minimum age subject to parental, religious or other forms of social approval, at which a person is legitimately allowed for marriage. Age and other prerequisites to ...

without parental consent except for

Nebraska Nebraska () is a state in the Midwestern region of the United States. It is bordered by South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Kansas to the south; Colorado to the southwe ...

(19), Mississippi and Puerto Rico (21). **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, it is the legal age to purchase a BBFC "18" rated film.

In sports

* In

association football Association football, more commonly known as football or soccer, is a team sport played between two teams of 11 players who primarily use their feet to propel the ball around a rectangular field called a pitch. The objective of the game is ...

(soccer), "the 18" is a slang term for the penalty area. * In

Australian rules football Australian football, also called Australian rules football or Aussie rules, or more simply football or footy, is a contact sport played between two teams of 18 players on an oval field, often a modified cricket ground. Points are scored by k ...

(except in AFL Women's), each team has 18 players on the field during play. * There are 18 holes on a regulation

golf Golf is a club-and-ball sport in which players use various clubs to hit balls into a series of holes on a course in as few strokes as possible. Golf, unlike most ball games, cannot and does not use a standardized playing area, and coping wi ...

course. * In

Nippon Professional Baseball or NPB is the highest level of baseball in Japan. Locally, it is often called , meaning ''Professional Baseball''. Outside Japan, it is often just referred to as "Japanese baseball". The roots of the league can be traced back to the formation ...

(Japanese baseball), No.18 is known as

Ace An ace is a playing card, Dice, die or domino with a single Pip (counting), pip. In the standard French deck, an ace has a single suit (cards), suit symbol (a heart, diamond, spade, or club) located in the middle of the card, sometimes large a ...

number.

Notes

{{Integers, zero Integers