18 (eighteen) is the

natural number In mathematics, the natural numbers are the numbers 0, 1, 2, 3, and so on, possibly excluding 0. Some start counting with 0, defining the natural numbers as the non-negative integers , while others start with 1, defining them as the positive in ...

following 17 and preceding 19. It is an even

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

Mathematics

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. ...

and an

abundant number In number theory, an abundant number or excessive number is a positive integer for which the sum of its proper divisors is greater than the number. The integer 12 is the first abundant number. Its proper divisors are 1, 2, 3, 4 and 6 for a total ...

. It is a

largely composite number A highly composite number is a positive integer that has more divisors than all smaller positive integers. If ''d''(''n'') denotes the number of divisors of a positive integer ''n'', then a positive integer ''N'' is highly composite if ''d''(' ...

, as it has 6

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 divisibl ...

s and no smaller number has more than 6 divisors. There are 18 one-sided

pentomino A pentomino (or 5-omino) is a polyomino of order 5; that is, a polygon in the Plane (geometry), plane made of 5 equal-sized squares connected edge to edge. The term is derived from the Greek word for '5' and "domino". When rotation symmetry, rota ...

es. In the

classification of finite simple groups In mathematics, the classification of finite simple groups (popularly called the enormous theorem) is a result of group theory stating that every List of finite simple groups, finite simple group is either cyclic group, cyclic, or alternating gro ...

, there are 18 infinite families of groups.

In science

Chemistry

* The 18-electron rule is a

rule of thumb In English language, 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 associat ...

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. The lanthanide and actinid ...

chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

for characterising and predicting the stability of metal complexes.

In religion and literature

* The

Hebrew Hebrew (; ''ʿÎbrit'') is a Northwest Semitic languages, Northwest Semitic language within the Afroasiatic languages, Afroasiatic language family. A regional dialect of the Canaanite languages, it was natively spoken by the Israelites and ...

word for "life" is ('' chai''), which has a

numerical value A number is a mathematical object used to count, measure, and label. The most basic examples are the natural numbers 1, 2, 3, 4, and so forth. Numbers can be represented in language with number words. More universally, individual numbers can ...

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 () is an Abrahamic religions, Abrahamic, Monotheism, monotheistic, ethnic religion that comprises the collective spiritual, cultural, and legal traditions of the Jews, Jewish people. Religious Jews regard Judaism as their means of o ...

, in the

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

;

Pirkei Avot Pirkei Avot (; also transliterated as ''Pirqei Avoth'' or ''Pirkei Avos'' or ''Pirke Aboth'', also ''Abhoth''), which translates into English as Chapters of the Fathers, is a compilation of the ethical teachings and maxims from Rabbinic Jewis ...

(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'' (, ) is a canopy under which a Judaism, Jewish couple stand during their Jewish wedding, wedding ceremony. It consists of a cloth or sheet, sometimes a tallit, stretched or supported over four poles, or sometimes manually held u ...

(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 The age of majority is the threshold of legal adulthood as recognized or declared in law. It is the moment when a person ceases to be considered a minor (law), minor, and assumes legal control over their person, actions, and decisions, thus te ...

). * 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'' (, ''Tefilat HaAmidah'', 'The Standing Prayer'), also called the ''Shemoneh Esreh'' ( 'eighteen'), is the central prayer of Jewish liturgy. Observant Jews recite the ''Amidah'' during each of the three services prayed on week ...

. * In

Ancient Roman In modern historiography, ancient Rome is the Roman people, Roman civilisation from the founding of Rome, founding of the Italian city of Rome in the 8th century BC to the Fall of the Western Roman Empire, collapse of the Western Roman Em ...

custom the number 18 can symbolise a

blood relative Consanguinity (from Latin '' consanguinitas'' 'blood relationship') is the characteristic of having a kinship with a relative who is descended from a common ancestor. Many jurisdictions have laws prohibiting people who are closely related by blo ...

As lucky or unlucky number

* In

Chinese tradition Chinese culture () is one of the world's earliest cultures, said to originate five thousand years ago. The culture prevails across a large geographical region in East Asia called the Sinosphere as a whole and is extremely diverse, with custo ...

, 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 The age of majority is the threshold of legal adulthood as recognized or declared in law. It is the moment when a person ceases to be considered a minor (law), minor, and assumes legal control over their person, actions, and decisions, thus te ...

, in which a minor becomes a legal adult. It is also the

voting age A legal voting age is the minimum age that a person is allowed to Voting, vote in a democracy, democratic process. For General election, general elections around the world, the right to vote is restricted to adults, and most nations use 18 year ...

marriageable age Marriageable age is the minimum legal age of marriage. Age and other prerequisites to marriage vary between jurisdictions, but in the vast majority of jurisdictions, the marriageable age as a right is set at the age of majority. Nevertheless, ...

drinking age The legal drinking age is the minimum age at which a person can legally consume alcoholic beverages. The minimum age alcohol can be legally consumed can be different from the age when it can be purchased in some countries. These laws vary betwee ...

and

smoking age The smoking age is the minimum legal age required to purchase or use tobacco or cannabis (drug), cannabis products. Most countries have laws that forbid sale of tobacco products to persons younger than certain ages, usually the age of majority. ...

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 any web page whose content is identified by a common domain name and is published on at least one web server. Websites are typically dedicated to a particular topic or purpose, such as news, education ...

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 (21 under federal law). **

Marriageable age Marriageable age is the minimum legal age of marriage. Age and other prerequisites to marriage vary between jurisdictions, but in the vast majority of jurisdictions, the marriageable age as a right is set at the age of majority. Nevertheless, ...

without parental consent except for

Nebraska Nebraska ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders South Dakota to the north; Iowa to the east and Missouri to the southeast, both across the Missouri River; Ka ...

(19),

Mississippi Mississippi ( ) is a U.S. state, state in the Southeastern United States, Southeastern and Deep South regions of the United States. It borders Tennessee to the north, Alabama to the east, the Gulf of Mexico to the south, Louisiana to the s ...

, (19) and Puerto Rico (21). **The minimum age at which one can purchase, rent, or buy tickets to

NC-17 The Motion Picture Association film rating system is used in the United States and its territories to rate a motion picture's suitability for certain audiences based on its content. The system and the ratings applied to individual motion picture ...

-rated films or buy video games with an Adults Only rating. *In the UK, 18 is the legal age to purchase a

BBFC The British Board of Film Classification (BBFC) is a non-governmental organisation founded by the British film industry in 1912 and responsible for the national classification and censorship of films exhibited at cinemas and video works (su ...

"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 Association football, more commonly known as football or soccer, is a team sport played between two teams of 11 Football player, players who almost exclusively use their feet to propel a Ball (association football), ball around a rectangular f ...

(soccer), "the 18" is a slang term for the

penalty area The penalty area or 18-yard box (also known less formally as the penalty box or simply box) is an area of an association football pitch. It is rectangular and extends to each side of the goal and in front of it. If any part of the ball is ove ...

. * In

Nippon Professional Baseball is a professional baseball league and the highest level of baseball in Japan. Locally, it is often called , meaning simply ''Professional Baseball''; outside of Japan, NPB is often referred to as "Japanese baseball". The roots of the league ...

(Japanese baseball), No.18 is known as the

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

number.

References

{{Integers, zero Integers